derive: refactor parser

`parser.rs` was a single file containing almost 2000 lines.

This PR split the file into multiple, smaller files. `Expr`, `Node`, and
`Target` each get an own file. Each struct gets a `parse()` method that
return `Result<Self>`, and every other detail is private to the file.

This PR should make this essential part of Askama more easy to
understand, and make future modifications easier.