Lexer- and parser-relative require resolution

[jgellin-sf]

This PR fixes the vmRequire functions for reading the lexer and parser during Typescript declaration generation. The git history indicates that problems in this area are due to slight misuse of the require.resolve function which returns a path string, not a module. Later changes worked around this problem in the case of the parser but limited the require to 'antlr4/index', 'antlr4', or the parserFile itself. Therefore any custom imports in the antlr grammar (for example, any require statement in the @header section) could not be loaded, leading to generation problems.
This PR simplifies require resolution by using require.resolve relative to the lexer and parser file to return the module path, and then require-ing that path. This should work in a general way, eliminating the need for the various conditions in vmRequire.

[jgellin-sf]

This PR should partially fix #32. The Typescript will generate, but the resulting parser declaration extends Parser, not the class name specified in the superClass option.

[jgellin-sf]

@mcchatman8009 I have some work dependent on this PR. I was wondering if you could comment on the timeline for merging/publishing it. If you won't have a chance for awhile that's ok with me. I'll publish a version to my local NPM registry, but I'd like to know when I can plan to integrate a version of the real tool. Thanks for this tool.

[mcchatman8009]

@jgellin-sf I'll review later today.

[jgellin-sf]

@mcchatman8009 Checking in on this PR.