feat(datetime): add parts for calendar day, active, and today (#27641)

Issue number: resolves #25340

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- Exposes the following parts for a calendar day: `calendar-day`,
`today`, and `active`
- Combines the `calendar-day-highlight` element with the `calendar-day`
element so developers don't have to know to style two different elements
& we don't have to expose them as separate parts
- Improves height parity of the calendar day across browsers
- Updates the `custom` e2e test to include an example of styling days
using the newly exposed CSS parts
- Adds tests for the focus states of the calendar day