[XKit] Fix creating NSTextLists with custom formats in .NET. Fixes #15766.

[rolfbjarne]

In .NET, we changed the API for creating a NSTextList with a specified
NSTextListMarkerFormats, making NSTextListMarkerFormats a strongly typed enum,
and not allowing any other value except those in NSTextListMarkerFormats.

This was a mistake, because NSTextList can be created with other format values
than those available in NSTextListMarkerFormats.

So fix this by:

• Adding another NSTextListMarkerFormats enum value that specifies that the
  actual format is a custom one (NSTextListMarkerFormats.CustomString).
• Resurface an 'NSTextListMarkerFormats(string)' constructor that can be used
  to create an NSTextListMarkerFormats with a custom string.
• Add a NSTextListMarkerFormats.CustomMarkerFormat property that always
  retrieves the underlying string value for the format.
• Add a NSTextListOptions.None enum value, which means no options (since
  NSTextListOptions is a set of flags, it should be possible to specify no
  flags).
• Add two convenience constructors that don't take a NSTextListOptions value,
  defaulting to NSTextListOptions.None.
• Add tests!

Fixes #15766.

[vs-mobiletools-engineering-service2]

✅ API diff for current PR / commit

Legacy Xamarin (No breaking changes)

• iOS: vsdrops gist (No breaking changes)
• tvOS: vsdrops gist (No breaking changes)
• watchOS (no change detected)
• macOS: vsdrops gist (No breaking changes)

.NET (No breaking changes)

• iOS: vsdrops gist (No breaking changes)
• tvOS: vsdrops gist (No breaking changes)
• MacCatalyst: vsdrops gist (No breaking changes)
• macOS: vsdrops gist (No breaking changes)

✅ API diff vs stable

Legacy Xamarin (No breaking changes)

• iOS: vsdrops gist (No breaking changes)
• tvOS: vsdrops gist (No breaking changes)
• watchOS: vsdrops gist (No breaking changes)
• macOS: vsdrops gist (No breaking changes)

.NET (No breaking changes)

• iOS: vsdrops gist (No breaking changes)
• tvOS: vsdrops gist (No breaking changes)
• MacCatalyst: vsdrops gist (No breaking changes)
• macOS: vsdrops gist (No breaking changes)
• Microsoft.iOS vs Microsoft.MacCatalyst: vsdrops gist

Legacy Xamarin (stable) vs .NET

• iOS: vsdrops gist
• tvOS: vsdrops gist
• macOS: vsdrops gist

ℹ️ Generator diff

Generator Diff: vsdrops (html) vsdrops (raw diff) gist (raw diff) - Please review changes)

Pipeline on Agent
Hash: 53c8e1ae104098bc15315b91d312fd6ab3c979c4 [PR build]

[vs-mobiletools-engineering-service2]

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Hash: 53c8e1ae104098bc15315b91d312fd6ab3c979c4 [PR build]

[vs-mobiletools-engineering-service2]

💻 [PR Build] Tests on macOS M1 - Mac Big Sur (11.5) passed 💻

✅ All tests on macOS M1 - Mac Big Sur (11.5) passed.

Pipeline on Agent
Hash: 53c8e1ae104098bc15315b91d312fd6ab3c979c4 [PR build]

[vs-mobiletools-engineering-service2]

🚀 [CI Build] Test results 🚀

Test results

✅ All tests passed on VSTS: simulator tests.

🎉 All 223 tests passed 🎉

Tests counts

✅ bcl: All 69 tests passed. Html Report (VSDrops) Download
✅ cecil: All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests: All 1 tests passed. Html Report (VSDrops) Download
✅ fsharp: All 7 tests passed. Html Report (VSDrops) Download
✅ framework: All 8 tests passed. Html Report (VSDrops) Download
✅ generator: All 2 tests passed. Html Report (VSDrops) Download
✅ interdependent_binding_projects: All 7 tests passed. Html Report (VSDrops) Download
✅ install_source: All 1 tests passed. Html Report (VSDrops) Download
✅ introspection: All 8 tests passed. Html Report (VSDrops) Download
✅ linker: All 65 tests passed. Html Report (VSDrops) Download
✅ mac_binding_project: All 1 tests passed. Html Report (VSDrops) Download
✅ mmp: All 2 tests passed. Html Report (VSDrops) Download
✅ mononative: All 12 tests passed. Html Report (VSDrops) Download
✅ monotouch: All 23 tests passed. Html Report (VSDrops) Download
✅ msbuild: All 2 tests passed. Html Report (VSDrops) Download
✅ mtouch: All 1 tests passed. Html Report (VSDrops) Download
✅ xammac: All 3 tests passed. Html Report (VSDrops) Download
✅ xcframework: All 8 tests passed. Html Report (VSDrops) Download
✅ xtro: All 2 tests passed. Html Report (VSDrops) Download

Pipeline on Agent
Hash: 53c8e1ae104098bc15315b91d312fd6ab3c979c4 [PR build]