refactor: add parse modes type hints, minor refactoring

[evermake]

Changes made:

• wellKnownParseModesMap map was replaced with knownParseModes object;
• buildTransformer (a.k.a. parseMode) arrow function was transformed to regular function and also was overloaded to show type hints of available parse modes;
• src/transformer.ts file formatted with Deno.

[evermake]

@KnorpelSenf , I've simplified code. Could someone review?

Also, is it okay, that deno check found 5 type errors here?

[KnorpelSenf]

Also, is it okay, that deno check found 5 type errors here?

No. Please fix them :)

[KnorpelSenf]

@KnightNiwrem this may be the day to set up CI for your repository :D

[evermake]

Also, is it okay, that deno check found 5 type errors here?

No. Please fix them :)

I guess they were before me... I haven't figured out their reasons yet.

[KnorpelSenf]

LGTM after type errors and potential linting errors are fixes.

The only question I have: are we even allowed to change the type from string to ParseMode? I think this is a breaking change because it will prevent existing code from compiling. That would mean that this PQ would require a new major version. I am not sure if @KnightNiwrem is fine with that.

If not, we can consider adding a helper type that grammY uses.

type StringWithSuggestions<S extends string> = (string & {}) | S; // permits `string` but gives hints

[KnorpelSenf]

@evermake are you still interested in fixing this up?