Mini-rust in Rust 017: Handling float literals

We're still on a mission to do some type-checking, and our plan is to allow writing `let x: i32 = 3.0;`, then produce a type error because `3.0` is not an integer that can be stored in an `i32`. The last piece of the puzzle is to be able to handle the `3.0` bit - i.e. to be able to lex, parse and evaluate floating-point numbers. More info: https://artificialworlds.net/blog/2025/02/28/mini-rust-in-rust-017-lex-parse-eval-floats