Add ReadOnlySpan string-like StartsWith and EndsWith API with globalization support...

Add ReadOnlySpan string-like StartsWith and EndsWith API with globalization support (#16223)

* Add ReadOnlySpan string-like StartsWith API with globalization support

* Add ReadOnlySpan string-like EndsWith API with globalization support

* Fix nits from PR feedback.

* Address PR feedback

* Add unsafe keyword in CompareInfo.Unix methods.

* Address PR feedback and fix signature of Interop.Globalization.EndsWith

* Code formatting cleanup as suggested by VS.

* Revert "Code formatting cleanup as suggested by VS."

This reverts commit 20931fbf1ae113c6acaa3a4b2c0789b46d7ab06b.

* Creating Span specific methods and working around use of IsFastSort

* Code cleanup

* Improve performance by avoiding slicing and fix impl bug

* Remove commented out code and revert some calls to FindString

* Updating checks for Unix to mimic what IsFastSort does.

* Add length checks and include IsFastSort check within the loop

* Fix bounds checks to not include 0x80

* Call correct method when StringComparison is IgnoreCase.

* Update exception type in string compare to remain consistent