feat(datetime-button): add base styles and examples

[liamdebeasi]

Pull request checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
  • Some docs updates need to be made in the ionic-docs repo, in a separate PR. See the contributing guide for details.
• Build (npm run build) was run locally and any changes were pushed
• Lint (npm run lint) has passed locally and any fixes were made for failures

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

Issue URL: Internal ticket.

What is the new behavior?

• Added sample index.html files. Not all of them are fully functional as this PR is mainly responsible for laying the groundwork for future PRs.
• Added the base styling for datetime-button
• Extracted the presentation types to a new DatetimePresentation type

Does this introduce a breaking change?

• Yes
• No

Other information

[sean-perkins]

Few questions in this area:

1. What does the // STUBs comment signify?
2. Do these state values get set to undefined at some point in execution/configuration?
3. Should these variables have default values?

[liamdebeasi]

"stub" means that the values are placeholders. It allows me to focus on getting the styles working first without having to worry about implementation details. The implementation is here: #25620 (note that the stubs are removed and replaced with actual values)