Light up core ASCII.Utility methods with Vector256/Vector512 code paths. (#88532)

* Lib upgrade for ToUtf16

* Upgrade NarrowUtf16ToAscii with Vector512

* Complete the upgrade in NarrowUtf16ToAscii method
with Vector512 and Vector256 APIs.

* Adding VectorXX paths to `GetIndexOfFirstNonAscii` functions.

* Adding optimization to Vecto256 VectorContainsNonAsciiChar method.

* Code path refactoring and cleanup.

* Code changes based on the review:
1. turn some variables into explicitly specified const.
2. removed some helper functions and inlined them.

* Resolve comments

* revert the changes at GetIndexOfFirstNonAsciiByte

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Co-authored-by: Ruihan-Yin <ruihan.yin@intel.com>

Author: anthonycanino