Regex: reduce allocation slightly, add tests, code cleanup, add parser comments ...

Regex: reduce allocation slightly, add tests, code cleanup, add parser comments (#30632)

* RegexParser & optionsstack ref

* Add test coverage for Group.Synchronized

* Adjust options mode test case

* Add inline comment '#' test branch

* Add comments

* Replace manual ToLower calls by Span.ToLower

* Make applicable fields readonly in parser

* Change to Assert to reduce an if check in one branch

* Code formatting

* Avoid string allocation when IgnoreCase set

Prefix patterns which are passed to RegexBoyerMoore are already
lowercased by the parser. Remove the redundant ToLower() call and assert
the patterns lowercase state

* Add surrogate pair positive & negative tests

* Add test cases for rtl anchor

Signed-off-by: dotnet-bot <dotnet-bot@microsoft.com>