Add SelectWidget support for grouped options and placeholder prop

[mnholtz]

What does this PR do?

• Part of https://github.com/pixiebrix/pixiebrix-app/issues/5209
• I need SelectWidget to be able to handle grouped options (it currently makes the assumption that the options prop isn't grouped, and thus passing grouped options breaks the selected option behavior)
• Also adding the placeholder prop while I'm add it, which we'll also need

Discussion

• TODO: Fix typing; the generics are going to need to be tweaked here

Demo

This change enables grouped options and custom placeholder text, e.g.:

Checklist

• Add jest or playwright tests and/or storybook stories
• Designate a primary reviewer @grahamlangford

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.43%. Comparing base (6d95047) to head (057670a).
Report is 71 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8468      +/-   ##
==========================================
- Coverage   73.47%   73.43%   -0.04%     
==========================================
  Files        1334     1352      +18     
  Lines       41259    41562     +303     
  Branches     7686     7799     +113     
==========================================
+ Hits        30316    30523     +207     
- Misses      10943    11039      +96     

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[github-actions]

Playwright test results - MV2

 44 passed
 12 skipped

Details

 Open report ↗︎
 56 tests across 19 suites
 11 minutes, 57 seconds
 057670a

Skipped tests

chrome › tests/extensionConsoleActivation.spec.ts › can activate a mod with built-in integration
chrome › tests/extensionConsoleActivation.spec.ts › can activate a mod via url
edge › tests/extensionConsoleActivation.spec.ts › can activate a mod with built-in integration
edge › tests/extensionConsoleActivation.spec.ts › can activate a mod via url
chrome › tests/regressions/sidebarLinks.spec.ts › #8206: clicking links from the sidebar doesn't crash browser
edge › tests/regressions/sidebarLinks.spec.ts › #8206: clicking links from the sidebar doesn't crash browser
chrome › tests/runtime/sidebarController.spec.ts › sidebar controller › shows focus dialog in top-level frame
edge › tests/runtime/sidebarController.spec.ts › sidebar controller › shows focus dialog in top-level frame
chrome › tests/runtime/sidebarNavigation.spec.ts › sidebar mod panels are persistent during navigation
chrome › tests/runtime/sidebarNavigation.spec.ts › sidebar form and temporary panels are unavailable after navigation
edge › tests/runtime/sidebarNavigation.spec.ts › sidebar mod panels are persistent during navigation
edge › tests/runtime/sidebarNavigation.spec.ts › sidebar form and temporary panels are unavailable after navigation

[github-actions]

Playwright test results - MV3

 56 passed

Details

 Open report ↗︎
 56 tests across 19 suites
 16 minutes, 44 seconds
 057670a

[github-actions]

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[grahamlangford]

No need for TValue | null, since we can create type NullableOption = Option<string | null> and it just works