Enhance support for fsl/fsy files

Now that https://github.com/fsprojects/FsLexYacc/pull/144 is merged, we should be able to take an explicit dependency on the core logics for fslex and fsyacc. The AST structures provided for lex and yacc files in these libraries should enable us to lgiht up many LSP features for lex/yacc files, including:

* intellisense for parser rules
* provide syntax highlighting for the F# code inside a rule (by creating 'virtual documents' that contain the F# code from the TopCode + the code in a specific rule)
* go-to-definition for the parser rules
* semantic coloration of tokens

I expect that some rounds of enhancements might be necessary to the FsLexYacc project, as those projects don't seem to surface ranges for core concepts like `parser rule` or `token`.