Encode TargetPaths before writing them to the editorconfig

[captainsafia]

Address dotnet/aspnetcore#28794.

Source generators and analyzers use an editor config file to transfer relevant metadata between MSBuild and the compiler. In Razor, the editor config is populated with the TargetPaths of the output files like so:

[/Users/captainsafia/Verifications/TestFilesNames/Views/#Shared/Error.cshtml]
build_metadata.AdditionalFiles.TargetPath = /Views/#Shared/Error.cshtml
build_metadata.AdditionalFiles.CssScope =

If there are characters in the editor config that are delineated as special characters by the editor config spec, then the string processing will be borked for the filename. For example, a "#" anywhere in the filename will cause the rest of the filename to be ignored since # is interpreted as a comment character in editorconfig.

To work around this, we base64 encode the TargetPath property before writing it to disk to ensure that only alphanumeric characters are written to the editor config.

[/Users/captainsafia/Verifications/TestFilesNames/Views/#Shared/Error.cshtml]
build_metadata.AdditionalFiles.TargetPath = Vmlld3MvI1NoYXJlZC9FcnJvci5jc2h0bWw=
build_metadata.AdditionalFiles.CssScope =

Then we decode the properties when reading them.

Note: this is a temporary workaround until a more permanent solution is in place at the compiler level.

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[TanayParikh]

LGTM; just some clarifying questions for my understanding.

[TanayParikh]

Is this still required when we have the below !char.IsLetterOrDigit?

[captainsafia]

Not really -- I mostly kept it so that it was clear treating path separators differently.

[TanayParikh]

So this is essentially used as a cache key?

[captainsafia]

Yeah, this gets used as a the hint name for the generated SourceText. It's the filename that gets written to disk when the /p:EmitCompilerGeneratedFiles=true is set true and is used as the identifier for the generated file.