Support dotnet_diagnostic options for source generated files

[333fred]

Closes #82285, fixes #41171.

Implements the API proposal from #82285. We now support the following section:

[**/$generated$/**/*.cs]
dotnet_diagnostic.WARN0001.severity = none

The $generated$ path is considered to be relative to the project root. This means that a ** is necessary at the start of many $generated$ paths, since many .editorconfigs live above a project root, not within the project root. Under the project root, the standard generator path is used. This is usually the name of the dll the generator came from, followed by the specific generator in question, followed by the hint name for the file the generator chose. For example, for a generator project/assembly named MyCoolGenerator, a generator inside named GeneratesCoolThings, and a hint name of CoolThing.g.cs, the path from the consuming project root would be $generated$/MyCoolGenerator/MyCoolGenerator.GeneratesCoolThings/CoolThing.g.cs.

[dotnet-policy-service]

This PR modifies public API files. Please follow the instructions at https://github.com/dotnet/roslyn/blob/main/docs/contributing/API%20Review%20Process.md for ensuring all public APIs are reviewed before merging.