Added RubyLexer::SIMPLE_SSTRING_RE to clean up the huge case stmt.
Added RubyLexer::in_arg_state? to refactor a bunch of patterns.
Added RubyLexer::space_vs_beginning to refactor ** vs *.
Cleaned up more code with result.

[git-p4: depot-paths = "//src/ruby_parser/dev/": change = 8871]