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

Issue number: Part of #28300 

---------

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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/test/checkbox.spec.ts

@@ -2,6 +2,23 @@ import { newSpecPage } from '@stencil/core/testing';
 
 import { Checkbox } from '../checkbox';
 
+describe('ion-checkbox: shadow parts', () => {
+  it('should render the checkbox with shadow parts', async () => {
+    const page = await newSpecPage({
+      components: [Checkbox],
+      html: `
+        <ion-checkbox>Checkbox</ion-checkbox>
+      `,
+    });
+
+    const checkbox = page.body.querySelector('ion-checkbox')!;
+
+    expect(checkbox).toHaveShadowPart('container');
+    expect(checkbox).toHaveShadowPart('label');
+    expect(checkbox).toHaveShadowPart('mark');
+  });
+});
+
 describe('ion-checkbox: disabled', () => {
   it('clicking disabled checkbox should not toggle checked state', async () => {
     const page = await newSpecPage({