Use vectorized T=byte implementations to optimize all MemoryExtensions APIs for T...

Use vectorized T=byte implementations to optimize all MemoryExtensions APIs for T != byte (#28080)

* Adding IsTypeNumeric helper

* Add more NUint operations and use IsTypeNumeric everywhere.

* Revert addition of LangVersion 7.2

* Fix formatting

* Revert use of nuint and IsNumericType for *IndexOf* APIs

* Fix comment, undo leftover changes, and fix indentation.

* Address PR feedback - use nuint where possible.

* PR feedback - Cleanup SequenceEqual just like SequenceCompareTo

* Add new NUInt operations for netcoreapp/coreclr mirror.

* Address PR feedback

* Add T = char and T = long tests for StartsWith and EndsWith

Signed-off-by: dotnet-bot-corefx-mirror <dotnet-bot@microsoft.com>