feat(checkbox): add shadow part for label (#28604)

Issue number: Part of #28300 

---------

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## What is the current behavior?
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Developers are unable to adjust margin, width, etc. of the checkbox
label

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

- Checkbox label has a shadow part.

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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and migration path for existing applications below. -->


## Other information

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screenshots of how the component looks before and after the change. -->
As part of this work, I investigated moving `pointer-events: none` up
the DOM tree so developers wouldn't be able to override it with the
shadow part. In my testing, I was unable to see any difference in
behavior with vs without `pointer-events: none`. Therefore, I removed it
entirely.

core/src/components/checkbox/checkbox.tsx

@@ -18,6 +18,7 @@ import type { CheckboxChangeEventDetail } from './checkbox-interface';
  * @slot - The label text to associate with the checkbox. Use the "labelPlacement" property to control where the label is placed relative to the checkbox.
  *
  * @part container - The container for the checkbox mark.
+ * @part label - The label text describing the checkbox.
  * @part mark - The checkmark used to indicate the checked state.
  */
 @Component({
@@ -284,6 +285,7 @@ export class Checkbox implements ComponentInterface {
               'label-text-wrapper': true,
               'label-text-wrapper-hidden': el.textContent === '',
             }}
+            part="label"
           >
             <slot></slot>
           </div>