Port System.Memory performance improvements to Convert.TryFromBase64Chars (#17033)

Port System.Memory performance improvements to Convert.TryFromBase64Chars (#17033)

* Copy over Base64.DecodeFromUtf8 verbatim

* Don't need the InPlace api

* Change input from Utf8 to Utf16

* isFinalBlock will always be true, so constant fold it through

* I only need a true/false result, so get rid of OperationStatus

* Rename variables to reflect Utf8->Utf16 change

* Update comments and some minor cleanup

* Make Convert use Base64 as a fast-path

* Incorporate PR feedback.

* Delete FromBase64_Decode

* Use a single safe stackalloc

* Rename Base64.cs to Convert.Base64.cs

and make its contents a part of the Convert class.

We should eventually move all the Base64-related
stuff into this new file but not as part of this PR.
We want to be able to diff the implementation change
separately from the code movement.

* Thanks VS for the unasked for indent