Superficial downstream Typescript support

[StoneCypher]

You can fake support of Typescript for parsers without actually meaningfully supporting it, but still provide some value to downstream users

All that it takes in practice is adding a string for the output type

This could be supported as an argument in the options tuple, with basically no other changes

The end result of such a thing is that output from the parser would be easy to integrate into a typescript project, and would receive non-trivial (though still incomplete) typechecking

If I provided a PR that did this, and it was short and not awful, would it be considered?

[StoneCypher]

This was previously pegjs/pegjs#562

[hildjj]

Yes, I'm interested in this.

[hildjj]

(assuming backward-compat, opt-in, and clean enough syntax for its use that doesn't conflict with anything else that's on the table) :)

[StoneCypher]

Everything I do will expect backward compatability and either opt-in or safe default. I am an absolutist about backcompat.

I will try my best to not conflict with other things, but I would appreciate help in that regard.