Move to more consistent patterns in the parser. (#65724)

* Improve parser consistency

* Simplify more

* Improve parser consistency

* Improve parser consistency

* simplify further

* Lint

* Simplify

* Simplify

* Update src/Compilers/CSharp/Portable/Parser/LanguageParser.cs

Co-authored-by: Jan Jones <[email protected]>

* Simplify more

* Use pattern

* Simplify

* Simplify

* Simplify

* Simplify

* Simplify

* Simplify

* Simplify

* Simplify

* Simplify

* Simplify

* Simplify

* Simplify

* Simplify

* Simplify

* Make static

* Make static

* Simplify more

* Simplify more

* Simplify more

* Simplify more

* Simplify more

* Simplify more

* Simplify more

* Simplify more

* Simplify more

* Simplify more

* Simplify more

* Simplify more

* Simplify more

* Simplify more

* Simplify more

* Simplify more

* Simplify more

* named arg

* Named args

* Named args and cleanup

* Cleanup

* Cleanup

Co-authored-by: Jan Jones <[email protected]>