va-checkbox | Add internal description slot

[Mottie]

Chromatic

https://102160-cb-slot--65a6e2ed2314f7b8f98609d8.chromatic.com

Description

Adding an internal description slot to the va-checkbox to allow adding formatted content inside a tiled checkbox to better match the design of our form's issue page (see screenshot):

• Our app requires multiple lines and styled text inside the tile description, so a slot was the best fix
• Moved the part="label" to a span that wraps the label to allow styling only the label text
• Note: if a <p> is used inside the slot, the font doesn't match the other description content - I didn't see where in the CSS this was being set (see third example in screenshot)
• Note 2: I noticed that if a description is added to va-checkbox, it prevents rendering of any slot="description" content.

See department-of-veterans-affairs/va.gov-team#102160

QA Checklist

• Component maintains 1:1 parity with design mocks
• Text is consistent with what's been provided in the mocks
• Component behaves as expected across breakpoints
• Accessibility expert has signed off on code changes (if applicable. If not applicable provide reason why)
• Designer has signed off on changes (if applicable. If not applicable provide reason why)
• Tab order and focus state work as expected
• Changes have been tested against screen readers (if applicable. If not applicable provide reason why)
• New components are covered by e2e tests; updates to existing components are covered by existing test suite
• Changes have been tested in vets-website using Verdaccio (if applicable. If not applicable provide reason why)

Screenshots

Acceptance criteria

• QA checklist has been completed
• Screenshots have been attached that cover desktop and mobile screens

Definition of done

• Documentation has been updated, if applicable
• A link has been provided to the originating GitHub issue (or connected to it via ZenHub)