Stop leaking identifiers in parser generator and use more DSLv2 types

[Xanewok]

Part of #638, ticks the "stop leaking identifiers" box

• We leverage the existing codegen_language_definition::model::Identifier in PG rather than rely on &'static strs from DSL v1, so we don't leak anymore and we start using the DSLv2 type in PG
• Similarly, we move some of the old DSLv1 utilities in favor of directly using the DSLv2 counterparts
• Version-dependent codegen is now extracted into a dedicated module rather than split ad-hoc in trie.rs and parser_definition.rs
• We separate the accumulator-specific state from the model itself to better convey what is required and necessary during the accumulation and what data is finally being derived from the accumulation and stored in the resulting ParserModel used in the MVC fashion for PG codegen

The remaining work is to stop using the DSLv1 Grammar(Visitor) but rather directly collect items from and use the DSLv2 model to construct the final ParserModel for codegen.

I wouldn't look too much into details like duplicated doc comments for the state accumulator/model because it's going away in the next PR, similarly naming for stuff like Versioned(Quote) that's also renamed temporarily and will be changed/removed since we will move off the DSLv1 model.

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[OmarTawfik]

I suggest removing the as _ from import statements, to enable unused warnings when it is no longer needed.

[Xanewok]

That's not true: after removing the usages I still get correct unused import: `Versioned as _` rustc diagnostic; like I mentioned, this is somewhat special-cased by rustc.

[OmarTawfik]

this is somewhat special-cased by rustc

hmmm, this is surprising. it goes against how underscore behaves in other places! thanks for pointing out 🤔
I wonder what is the benefit of that syntax then, rather than just use Versioned;? in this case, the underscore is meaningless to rustc?

[Xanewok]

It exists when you want to import a trait but also use a type with the same name - in this case, using the trait in scope allows you to use the trait methods but you can still use the other data type with a given name.

[OmarTawfik]

Thanks!