Light up Ascii.Utility methods with Vector512 code paths. #89280

This issue tracks upgrades to Ascii.Utility with Vector512 (and in some cases Vector256) code paths.

We plan to upgrade the following methods (the ones checked off have already been merged into main).

• NarrowUtf16ToAscii
• WidenAsciiToUtf16
• GetIndexOfFirstNonAsciiChar
• GetIndexOfFirstNonAsciiByte
• IsValidCore
• Equals/EqualsIgnore
• CaseConversion