fix(select): optgroups are not visible to screen readers

[Splaktar]

PR Checklist

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• The commit message follows our guidelines
• Tests for the changes have been added or this is not a bug fix / enhancement
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PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Enhancement
[ ] Documentation content changes
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Option groups are not read by screen readers.

Issue Number:
#11240

What is the new behavior?

Option groups are read by screen readers without breaking the reading of indexes.

• add single selection optgroup demo
• add tests for optgroup aria-label
• add tests for optgroup options' aria-setsize and aria-posinset

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

On VoiceOver for macOS, indexes are no longer announced if md-optgroups are used but it's probably better to have the groups announced than to worry about indexes being announced.

For ChromeVox, both the option groups and indexes are announced properly.

This PR is based on PR #11761 and can't be merged or presubmitted until that PR is merged and sync'd.

[mmalerba]

LGTM