feat: add high contrast themes (#29010)

⚠️ This is a combination of previously approved PRs with the
exception of
fe9dca513c.
This change was made as a result of
https://github.com/ionic-team/ionic-framework-design-documents/pull/248.

Issue number: Internal

---------

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issue. -->

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## What is the current behavior?
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Users do not have a way of increasing the contrast in Ionic apps. This
is valuable for people with low vision as increasing the contrast
between foreground and background content helps improve readability.

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

- Adds a high contrast light and high contrast dark theme. As with our
other themes, developers can choose between system, class, and always
stylesheets.

While we aim to improve contrast for text and UI components, this
feature prioritizes text in the event that both text and UI component
cannot be improved without one negatively impacting the other.

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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## Other information

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screenshots of how the component looks before and after the change. -->

Dev build: `7.6.7-dev.11706894781.1cd59fde`

Testing instructions:

1. Open `src/themes/test/css-variables`. Activate the high contrast
light and dark themes to verify that contrast does increase.
2. Use the dev build to integrate the theme into a test app (conference
app, starter app, etc).

I'd recommend using these imports:

```css
@import "@ionic/angular/css/themes/high-contrast.system.css";
@import "@ionic/angular/css/themes/high-contrast-dark.system.css";
```
Note: Make sure this is imported **after** `core.scss`

---------

Co-authored-by: Shawn Taylor <shawn@ionic.io>
Co-authored-by: Brandy Carney <brandyscarney@users.noreply.github.com>
Co-authored-by: Sean Perkins <13732623+sean-perkins@users.noreply.github.com>
Co-authored-by: ionitron <hi@ionicframework.com>
Co-authored-by: Maria Hutt <thetaPC@users.noreply.github.com>
Co-authored-by: Amanda Johnston <90629384+amandaejohnston@users.noreply.github.com>

core/src/components/datetime-button/datetime-button.scss

@@ -23,7 +23,7 @@
 
   border: none;
 
-  background: var(--ion-color-step-300, #edeef0);
+  background: var(--ion-color-step-300, var(--ion-background-color-step-300, #edeef0));
 
   color: $text-color;