feat: add ForceInternal configuration option

[xC0dex]

Currently, the generated extension class is always public, if the enum is also public. This may not be entirely ideal in certain circumstances.

This PR adds support for controlling the accessibility of generated enum extension classes.
Extension accessibility can now be configured globally via the EnumGenerator_ForceInternal MSBuild property or per enum using the IsInternal flag on the [EnumExtensions] attribute.
By default, extensions follow the enum’s accessibility, but public enums can now explicitly generate internal extensions when needed.

This is one possible approach to implementing it. Feel free to edit or close the PR. 🙂

[andrewlock]

Thanks @xC0dex! I'd like to add some tests for this, but I'm happy to take it unless you'd like to 🙂 I'd like to add

• Update the SourceGenerationSnapshotTests to test the alternative geenration
• Add a unit tests snapshot for a forced-internal enum
• Add an integration test to force internal
  • This is probably the biggest pain, we'll likely need to add a new enum, because forcing internal means some of the tests won't work as written

I'm going to push out a new version of the generator very shortly, and I'm going to be OOO for a couple of weeks after, so this probably won't make it in, sorry!

[ScarletKuro]

Very excited for this feature to use in MudBlazor