Register FsLex/FsYacc items as UpToDateCheckInput and tidy

[nojaf]

FsLexYacc.targets did not register .fsl and .fsy files as build inputs, so an IDE fast up-to-date check could skip the build after a grammar edit. Add the same UpToDateCheckInput item group dotnet/fsharp uses. Fixes #242.

Date the 12.0.0 release notes entry, which is already on NuGet, and start a 12.0.1 section for this fix.

Build script and repository housekeeping that came out of verifying the change:

• Add a shared Restore stage to every pipeline so ./build.fsx works standalone. Only build.sh, build.cmd and CI ran dotnet tool restore and dotnet paket restore first, and .paket is gitignored.
• Stop tracking generated sources: the fslex/fsyacc outputs in FsLex.Core and FsYacc.Core and the AssemblyInfo.fs files, all of which build.fsx regenerates. Remove the orphaned AssemblyInfo.fs under FsLexYacc.Build.Tasks, which nothing compiles.
• Migrate FsLexYacc.sln to FsLexYacc.slnx and point the pack step at it.
• Drop build.fsx.lock and pin local tool versions with rollForward off.
• Fix FS3873 in fslexast.fs by wrapping the character range in seq.
• Suppress NU1510 for the test projects. Paket emits a PackageReference for the transitive System.Collections.Immutable, which the .NET 10 SDK prunes, and Paket cannot omit it.