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

Issue number: Part of #28300 

---------

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issue. -->

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## What is the current behavior?
<!-- Please describe the current behavior that you are modifying. -->
Developers are unable to adjust margin, width, etc. of the checkbox
label

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

@@ -90,13 +90,6 @@
 }
 
 .label-text-wrapper {
-  /**
-   * This ensures that double tapping this text
-   * clicks the <label> and focuses the checkbox
-   * when a screen reader is enabled.
-   */
-  pointer-events: none;
-
   text-overflow: ellipsis;
 
   white-space: nowrap;
@@ -173,7 +166,6 @@ input {
   opacity: 0;
 }
 
-
 // Justify Content
 // ---------------------------------------------
 
@@ -200,7 +192,6 @@ input {
   align-items: center;
 }
 
-
 // Label Placement - Start
 // ----------------------------------------------------------------
 
@@ -221,7 +212,6 @@ input {
   @include margin(null, $form-control-label-margin, null, 0);
 }
 
-
 // Label Placement - End
 // ----------------------------------------------------------------
 
@@ -242,7 +232,6 @@ input {
   @include margin(null, 0, null, $form-control-label-margin);
 }
 
-
 // Label Placement - Fixed
 // ----------------------------------------------------------------
 
@@ -317,7 +306,6 @@ input {
   opacity: 1;
 }
 
-
 // Disabled Checkbox
 // ---------------------------------------------