[Performance]: Add fast-paths for Char.IsDigit

[baronfel]

Issue Description

https://github.com/dotnet/sdk/pull/49328 raised the issue that Char.IsDigit isn't currently in the fast-paths.

We need to do work similar to https://github.com/dotnet/msbuild/pull/12025 to add a fast path handler for this static method so the SDK is only using optimized code paths.

Steps to Reproduce

See https://github.com/dotnet/sdk/pull/49328

Data

n/a

Analysis

No response

Versions & Configurations

No response

Regression

• [ ] yes
• [ ] no

Regression Details

No response