feat(input): add soft shape for ionic theme (#29477)

Issue number: internal

---------

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## What is the current behavior?
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The input component does not support the `soft` shape.

## What is the new behavior?
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this PR. -->

- `soft` has been added to shape, but will only work for the `ionic`
theme.
- Added tests

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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## Other information

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[Preview](https://ionic-framework-git-fw-6096-ionic1.vercel.app/src/components/input/test/shape?ionic:theme=ionic)

core/src/components/input/input.ionic.scss

@@ -104,6 +104,10 @@
 // Ionic Input Shapes
 // --------------------------------------------------
 
+:host(.input-shape-soft) {
+  --border-radius: #{$ionic-border-radius-rounded-medium};
+}
+
 :host(.input-shape-round) {
   --border-radius: #{$ionic-border-radius-rounded-full};
 }