Rename ruff_python_whitespace to ruff_python_trivia (#5886)

## Summary

This crate now contains utilities for dealing with trivia more broadly:
whitespace, newlines, "simple" trivia lexing, etc. So renaming it to
reflect its increased responsibilities.

To avoid conflicts, I've also renamed `Token` and `TokenKind` to
`SimpleToken` and `SimpleTokenKind`.

crates/ruff_python_ast/src/identifier.rs

@@ -14,7 +14,7 @@ use ruff_text_size::{TextLen, TextRange, TextSize};
 use rustpython_ast::{Alias, Arg, ArgWithDefault};
 use rustpython_parser::ast::{self, ExceptHandler, Ranged, Stmt};
 
-use ruff_python_whitespace::{is_python_whitespace, Cursor};
+use ruff_python_trivia::{is_python_whitespace, Cursor};
 
 use crate::source_code::Locator;