Consider `SearchValues` for known-at-compile-time character sets to feed into IndexOfAny

https://devblogs.microsoft.com/dotnet/performance-improvements-in-net-8/#searchvalues

We have several checks for things like "is this a valid property name" and "are there invalid path characters" that could benefit from the prebaked comparisons of `SearchValues`.

I took an early crack at this: https://github.com/dotnet/msbuild/commit/afc70de6143cfc3909a3dfed70383d8259db0864. It looks like net35 compatibility in some of the core functions will be an implementation problem--but the benefit may be worth writing a new implementation.