feat: improve TouchBarButton/TouchBarLabel accessibility

[codebytere]

Description of Change

Resolves #20443.

Add an optional accessibilityLabel property to TouchBarButtons and TouchBarLabels, such that when a button is created with a confusing or icon-only label, developers can pass a description to VoiceOver to describe its purpose.

After this change:

const spin = new TouchBarButton({
    label: '🎰',
    accessibilityLabel: 'Carousel Spinner',
    backgroundColor: '#7851A9',
    click: () => { console.log('I did a spin!') }
})

would be read by VoiceOver as "Carousel Spinner, button".

cc @MarshallOfSound

Checklist

• PR description included and stakeholders cc'd
• npm test passes
• relevant documentation is changed or added
• PR title follows semantic commit guidelines
• PR release notes describe the change in a way relevant to app developers, and are capitalized, punctuated, and past tense.

Release Notes

Notes: Improve TouchBarButton and TouchBarLabel a11y with accessibilityLabel property.

[jkleinsc]

Merging as CI failures unrelated to PR changes

[release-clerk]

Release Notes Persisted

Improve TouchBarButton and TouchBarLabel a11y with accessibilityLabel property.

[bpasero]

@codebytere quick question: would it be easy to add this property also to TouchBarSegmentedControl?