Complete revision of schema model and code generation

[carueda]

Is your feature request related to a problem? Please describe.

The implementation has become hard to maintain over time as features were added, not always in the most elegant way.

Describe the solution you'd like

A complete revision of the schema model and, subsequently, the code generation part.

Primary goals:

1. A more flexible schema definition structure, allowing additional features.

   • Ideally --longer term-- with a new schema grammar mainly to circumvent limitations imposed by using TS Config for purposes of parsing a schema (as opposed to a regular config)
   • However, initially, still using TS Config as much as possible to facilitate transition to the new implementation with no, or minimal changes, by clients
   • Optionally, facilitate only-type generation, i.e., not also including the extraction logic (the TS Config calls) in the generated source file. This would make possible the combination of tscfg-based type generation with additional elements to use some other tool for the extraction logic itself (e.g., with the generated types having pureconfig elements).

2. A simplified codebase that should be much easier to maintain and extend.