Add string.Contains shims for .NET Standard 2.0

[xtqqczze]

• Add C# 14 extension members to polyfill the following APIs:

  • String.Contains(Char)
  • String.Contains(String, StringComparison)

• Remove suppression of CA2249: Consider using 'string.Contains' instead of 'string.IndexOf'

[MihaZupan]

#119673 is also adding framework polyfills for new APIs.

My preference would be to get that reviewed first, so that we can use a common switch of including such helper files in projects.

I also had changes for Encoding and Span.Contains APIs sitting in a branch since they depend on the PR mentioned above being merged first:

• Span-based Encoding APIs MihaZupan@56d2ee7
• Span.Contains MihaZupan@e819ccb (effectively a dupe of what this PR is doing)

[MihaZupan]

@xtqqczze Do you think it's worth moving forward here given that MihaZupan@e819ccb already has a superset of these changes?

[xtqqczze]

@xtqqczze Do you think it's worth moving forward here given that MihaZupan@e819ccb already has a superset of these changes?

I don't see string.Contains methods in those changes, only ReadOnlySpan<T>.Contains. Looks like it touches many of the same files though.