Refactor: move JSON path generation logic in Generator

[georgesittas]

Addresses items (1) and (2) mentioned in the "to do" section of #2914.

Transpiling * is a little more nuanced and requires more investigation, because despite some dialects not supporting this syntax, I think it may still be possible to simulate it by simply omitting it. For example, in DuckDB you can do:

D SELECT '["a", "b", "c"]' -> '$[*]';
┌───────────────────────────────┐
│ ('["a", "b", "c"]' -> '$[*]') │
│            json[]             │
├───────────────────────────────┤
│ ["a", "b", "c"]               │
└───────────────────────────────┘

But in BigQuery this syntax is not supported, since they don't recognize the * inside the brackets ("Unsupported operator in JSONPath: *"). However, if we completely omit the brackets, it seems like the result matches:

> SELECT JSON_EXTRACT('["a", "b", "c"]', '$');
f0_
["a","b","c"]

The exact same thing holds e.g. for Trino. Not sure if we wanna solve this now, though.