Rename parser to lexer

Signed-off-by: Marcel Müller <neikos@neikos.email>

tests/checks.rs

@@ -5,7 +5,7 @@ fn check_files() {
     for file in files {
         let input = std::fs::read_to_string(file.unwrap().path()).unwrap();
 
-        let Ok(parsed) = nomo::parser::parse(input.into()) else {
+        let Ok(parsed) = nomo::lexer::parse(input.into()) else {
             continue;
         };