RubyParser: basic support for regex literals

[xavierpinho]

• Supports (potentially interpolated) regex literals in the parser
• RubyParser.g4: renamed interpolation as interpolatedStringSequence, to avoid any confusion with regex literals' own interpolation sub-production (now named interpolatedRegexSequence)
• RubyLexerBase: refactored the predicate for deciding when to start a regex literal by using "sentinel" tokens more explicitly. This approach will need to be reconsidered, as it can't be exhaustive. But fortunately this is contained exclusively in RubyLexer