Add an opt-in to write the generated lexer and parser to obj

[nojaf]

MSBuild only honours FileWrites under the output or intermediate folder, so the registration that was meant to let dotnet clean remove the generated .fs and .fsi never took effect: by default they land next to the grammar, outside both folders.

FsLexYaccOutputToIntermediate=true now defaults FsLexOutputFolder and FsYaccOutputFolder to IntermediateOutputPath, so clean removes the generated sources and they stay out of the source tree. Projects reference them as $(FsLexOutputFolder)Lexer.fs and $(FsYaccOutputFolder)Parser.fs(i). Because every existing project references Parser.fs next to Parser.fsy, this is opt-in for now and becomes the default in the next major version.

The defaults are resolved at evaluation time so the properties can be used in Compile items, which requires the targets to be imported after the SDK targets, as the NuGet package does. A project that imports the targets file from its body would otherwise get Compile items that silently point next to the grammar, so FsLexYaccCheckOutputFolders fails the build with an error that explains the cause and the fix.

JsonLexAndYaccExample opts in to cover the new mode, and imports the SDK explicitly so the targets evaluate the way a package consumer sees them. LexAndYaccMiniProject keeps the default so both modes stay covered.

Release as 12.2.0.

Closes #247

[github-advanced-security]

You are seeing this message because GitHub Code Scanning has recently been set up for this repository, or this pull request contains the workflow file for the Code Scanning tool.

What Enabling Code Scanning Means:

• The 'Security' tab will display more code scanning analysis results (e.g., for the default branch).
• Depending on your configuration and choice of analysis tool, future pull requests will be annotated with code scanning analysis results.
• You will be able to see the analysis results for the pull request's branch on this overview once the scans have completed and the checks have passed.

For more information about GitHub Code Scanning, check out the documentation.

[baronfel]

LGTM, and as we discussed I would just go ahead and change defaults to emit the files to the Intermediate locations. MSBuild Targets are (by convention) expected to not pollute the source tree because it can lead to circularity and all kinds of fun, subtle issues during the build.