Add TextEquals helper methods to Utf8JsonReader (dotnet/corefx#35979)

Add TextEquals helper methods to Utf8JsonReader (dotnet/corefx#35979)

* Add initial impl of ValueEquals with basic tests.

* Rename to TextEquals based on API review feedback.

* Move to separate test file, increase coverage, and fill in missing impl
details.

* Add xml comments and more test cases.

* Re-enable all the tests.

* Update tests to be netstandard compliant.

* Rename some existing tests to be clearer.

* Address PR feedback.

* Return early if we know unescaping won't produce a match.

* More eagerly check the lengths to return mismatch sooner.

* Add tests to improve code coverage and re-write to avoid unreachable
code.

* Fix resource string merge issue.

* Fix test build failures on netstandard (missing implicit string->span
cast)

Commit migrated from https://github.com/dotnet/corefx/commit/b8bc4ff80c5f7baa681e8a569d367356957ba78a