Add UnsafeRelaxedJsonEscaping static to JavascriptEncoder. (dotnet/corefx#39550)
* Add UnsafeRelaxedJsonEscaping static to JavascriptEncoder.
* Remove duplicate test case and fix test name.
* Fix formatting nits and move the new class to a separate file.
* No need to special case single quote, and fix more formatting.
* Update the remarks section to call out the difference between Default
and Relaxed.
Commit migrated from https://github.com/dotnet/corefx/commit/
30ece647dae7d9ddff8a24cdd615119fde259927