parser/refactor def (#2986)

* Move tests into own file

* Move data structs to own file

* Move functions parsing 1 Token (primitives) into own file

* Rename param_flag_list to signature

* Add tests

* Fix clippy lint

* Change imports to new lexer structure

crates/nu-parser/src/lib.rs

@@ -1,5 +1,7 @@
 #[macro_use]
 extern crate derive_is_enum_variant;
+#[macro_use]
+extern crate derive_new;
 
 mod errors;
 mod lex;