[css-ui] Pseudo-elements for checkmark and dropdown icon for appearance:base `<select>`

[mozilla-apprentice]

Resolutions were made for csswg-drafts/#10908.

[css-ui] Pseudo-elements for checkmark and dropdown icon for appearance:base `<select>`

• RESOLVED: add pseudo-elements for the select button and option checkmarks which are fully stylable pseudo-elements with content specified by the content property
• RESOLVED: Adopt ::check or ::checkmark for checkbox/radio checks

Discussion.

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To file a bug automatically for these resolutions, add the bug label to the issue.

If no bug is needed, the issue can be closed.

[dholbert]

We don't support appearance:base yet, so I think we can close without a bug.