feat(input): add rectangular shape for the ionic theme (#29476)

Issue number: internal

---------

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## What is the current behavior?
The input supports an undefined and a `round` shape.

## What is the new behavior?
Adds support for the `rectangular` shape for the `ionic` theme &
screenshot tests for this shape with the outline fill.

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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

@@ -55,8 +55,8 @@
 ::slotted([slot="end"]),
 .input-clear-icon {
   /**
-   * The target area has a z-index of 1, so the slotted elements 
-   * should be higher. Otherwise, the slotted elements will not 
+   * The target area has a z-index of 1, so the slotted elements
+   * should be higher. Otherwise, the slotted elements will not
    * be interactable. This is especially important for the clear
    * button, which should be clickable.
    */
@@ -108,6 +108,10 @@
   --border-radius: #{$ionic-border-radius-rounded-full};
 }
 
+:host(.input-shape-rectangular) {
+  --border-radius: #{$ionic-border-radius-square};
+}
+
 // Input Bottom Content
 // ----------------------------------------------------------------