Use proper (lexer) and parser

[sayanarijit]

This module is getting complicated. So far I've avoided using lexers and parsers to keep things simple.
However, I think it's time we dive into these advanced concepts.

TIP: See if we can avoid implementing our own lexer by using regex (I think we can) but without complicating things.

[sayanarijit]

Rust has a very good library called nom that lets us implement our own parser using a very intuitive API. Let's try using that. See pydantic-core for Rust-Python interop reference.