fix(select): do not collapse to width: 0 when placed in flex container (#28631)

Issue number: Internal

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## What is the current behavior?
<!-- Please describe the current behavior that you are modifying. -->

We currently apply a workaround to `ion-select` so it can wrap correctly
inside of `ion-item`:
357b8b2beb/core/src/components/select/select.scss (L99-L103)

However, this causes issues when a parent element has `display: flex`
because the `ion-select` width becomes 0.

## What is the new behavior?
<!-- Please describe the behavior or changes that are being added by
this PR. -->

- In order to get the desired behavior, we need the `ion-select` (and
other elements in the default slot) to either truncate or wrap within
its own container and then have the entire container (i.e. the entire
`ion-select`) wrap to the next line once the container is too small.

To achieve this, I needed to set a min-width on `.item-inner` to define
the point at which the element should wrap to the next line. I also
changed the flex basis from `auto` to `0` which means the initial main
size of the flex item will be 0px. In reality, this will be
`--inner-min-width` since we also set `min-width:
var(--inner-min-width)`. I used `0` for simplicity but I can change this
to use the CSS variable if that's more clear. Since we also set
`flex-grow: 1` we indicate that the element can grow from that basis
(but it cannot shrink).

I chose `--inner-min-width: 4rem` to minimize the number of diffs. We
can certainly change this, but it may cause some diffs as certain
elements will start wrapping sooner. I also chose to use `rem` because
having a fixed min-width means that fewer characters are going to fit in
the same space as text scales.

I made this a CSS variable but left it undocumented. If developers need
a way of changing this `min-width` they can request it and we can easily
expose this variable. However, I think `4rem` is small enough that this
should be sufficient.

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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and migration path for existing applications below. -->


## Other information

<!-- Any other information that is important to this PR such as
screenshots of how the component looks before and after the change. -->

The visual diffs here are correct. The table below shows the screenshot
group and an explanation for why the changes are correct.


| Path | Example | Details |
| - | - | - |
| `disabled` |
[Link](https://github.com/ionic-team/ionic-framework/pull/28631/files#diff-d529716f95f7a7aa82c88588104220775b728af67077f48cd47a8afa04423143)
| The searchbar is able to shrink slightly to fit on the same line as
the checkbox at the bottom. |
| `highlight` |
[Link](https://github.com/ionic-team/ionic-framework/pull/28631/files#diff-0b64f24c91393923701d1ced4e330a1c6b926d72ee461b8ab1e135e708be3457)
| We're changing how small the main content can get, so the input is
only wrapping once it gets to `--inner-min-width`. |
| `legacy/fill` |
[Link](https://github.com/ionic-team/ionic-framework/pull/28631/files#diff-2ef8dbfa5e69e2b96c3e1ed29ab962f08cf5ba2aaf2af773e40bd143e38a4bef)
| We're changing how small the main content can get, so the input is
only wrapping once it gets to `--inner-min-width`. |
| `slotted-inputs` |
[Link](https://github.com/ionic-team/ionic-framework/pull/28631/files#diff-2f173c7303969d6a6c58f30a618cebc3caf918d3761fc83df5642fd48dfabd7b)
| We're changing how small the main content can get, so the range is
only wrapping once it gets to `--inner-min-width`. |

`slotted-inputs` note: I'd argue many of these examples are not best
practices. For example, adding a range in the start slot and the end
slot is a bit unusual. I'm not aware of any native apps that implement
this pattern.

popover note: I [removed the `ion-item` from the `popover/test/async`
test](331fcb859c).
There was a diff because the min-width increased, but IMO that component
should not be used in the popover test since we want to test the
popover, not the item.

--------

Demo:

| `feature-7.6` | `branch` |
| - | - |
| <video
src="https://github.com/ionic-team/ionic-framework/assets/2721089/693d4947-fa33-460d-bc7f-7b96b6338032"></video>
| <video
src="https://github.com/ionic-team/ionic-framework/assets/2721089/df35ca73-87aa-4e76-9bb7-99f0f2810640"></video>
|

(In this demo I updated the `ion-select` to wrap within its own
container first instead of truncate. We may want to consider doing this
by default, but I think this is out of scope for this task)

---------

Co-authored-by: ionitron <hi@ionicframework.com>
Co-authored-by: Brandy Carney <brandy@ionic.io>

core/src/components/popover/test/async/index.html

@@ -40,9 +40,7 @@
       popover.addEventListener('ionMount', () => {
         popover.innerHTML = `
           <div style="padding: 10px;">
-            <ion-list>
-              <ion-item>Item 1</ion-item>
-            </ion-list>
+            Popover Content
           </div>
         `;
       });