[.NET] Fix interpolated strings in ScriptPropertyDefVal

[paulloz]

• Inject OpenParenToken and CloseParenToken around all ExpressionSyntax that are children of InterpolationSyntax in our FullQualifiedSyntax function
• Added some unit tests
• Fixes [.NET] Interpolated strings as exported member's default value break source generation #88982

[raulsntos]

Since we only care about the ScriptPropertyDefVal generator for these tests, we could add the exported fields to a separate file so we only need to add the GeneratedSources for *_ScriptPropertyDefVal.generated.cs. And then make a new test in ScriptPropertyDefValGeneratorTests.cs.

Also, do we want to add a test for nested string interpolation? For example: $"The quick brown fox jumps over {GD.VarToStr($"the lazy {Engine.GetVersionInfo()}")}";

[raulsntos]

LGTM. Thanks!

[akien-mga]

Thanks!