fix(input, textarea, select): account for multiple start/end slot elements (#29172)

Issue number: Internal

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## What is the current behavior?
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We add margin on content in the start/end slots so they do not run up
against either the visible text label of the text in the input. However,
we did not account for when multiple elements are placed into the same
slot. As a result, this margin is added more times than it needs to be.

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

- Updated the selector so the margin is only applied to either the last
or first element in the slot.

## 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. -->


Test Steps: 

1. Check out `feature-8.0`
2. Add multiple `ion-button` elements into either the start or end slot
on an input (you can reuse the template in
`src/components/input/test/slots`
3. Observe that margin is added to every `ion-button`.

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Co-authored-by: ionitron <[email protected]>

Author: liamdebeasi

core/src/components/input/input.scss

@@ -596,12 +596,12 @@
 // Start/End Slots
 // ----------------------------------------------------------------
 
-::slotted([slot="start"]) {
+::slotted([slot="start"]:last-of-type) {
   margin-inline-end: $form-control-label-margin;
   margin-inline-start: 0;
 }
 
-::slotted([slot="end"]) {
+::slotted([slot="end"]:first-of-type) {
   margin-inline-start: $form-control-label-margin;
   margin-inline-end: 0;
 }

core/src/components/input/test/slot/input.e2e.ts

@@ -8,9 +8,13 @@ configs().forEach(({ title, screenshot, config }) => {
         `
           <ion-input label-placement="start" fill="solid" value="100" label="Weight" clear-input="true">
             <ion-icon slot="start" name="barbell" aria-hidden="true"></ion-icon>
+            <ion-icon slot="start" name="heart" aria-hidden="true"></ion-icon>
             <ion-button slot="end" aria-label="Show/hide password">
               <ion-icon slot="icon-only" name="lock-closed" aria-hidden="true"></ion-icon>
             </ion-button>
+            <ion-button slot="end">
+              <ion-icon slot="icon-only" name="trash" aria-hidden="true"></ion-icon>
+            </ion-button>
           </ion-input>
         `,
         config
@@ -25,9 +29,13 @@ configs().forEach(({ title, screenshot, config }) => {
         `
           <ion-input label-placement="floating" fill="solid" value="100" label="Weight" clear-input="true">
             <ion-icon slot="start" name="barbell" aria-hidden="true"></ion-icon>
+            <ion-icon slot="start" name="heart" aria-hidden="true"></ion-icon>
             <ion-button slot="end" aria-label="Show/hide password">
               <ion-icon slot="icon-only" name="lock-closed" aria-hidden="true"></ion-icon>
             </ion-button>
+            <ion-button slot="end">
+              <ion-icon slot="icon-only" name="trash" aria-hidden="true"></ion-icon>
+            </ion-button>
           </ion-input>
         `,
         config