Conversion of a specific start symbol (instead of all)

[sungshik]

Currently, it isn't possible to indicate that only a specific start symbol---instead of all start symbols---of an input Rascal grammar should be converted to a TextMate grammar when calling toTmGrammar. One way in which this can cause problems is when a Rascal grammar defines multiple start symbols with disjoint sets of (ci)literals (details: usethesource/rascal-language-servers#591).

For now, the workaround is for users to first slice the input Rascal grammar for a specific start symbol themselves before passing it toTmGrammar. A more user-friendly solution would be to extend toTmGrammar with a new keyword argument to indicate the start symbol and do the slicing internally.