Faster tokenizer lookahead (#13341)

* refactor: simplify token context structure

* add benchmark

* perf: return a sub-state on tokenizer lookahead

* Update packages/babel-parser/src/tokenizer/index.js

Co-authored-by: Brian Ng <bng412@gmail.com>

* Update packages/babel-parser/src/tokenizer/index.js

Co-authored-by: Brian Ng <bng412@gmail.com>

* remove irrelevant comment

* fix: guard curPosition with isLookahead

* add test cases

Co-authored-by: Brian Ng <bng412@gmail.com>

packages/babel-parser/src/tokenizer/state.js

@@ -178,3 +178,14 @@ export default class State {
     return state;
   }
 }
+
+export type LookaheadState = {
+  pos: number,
+  value: any,
+  type: TokenType,
+  start: number,
+  end: number,
+  /* Used only in readSlashToken */
+  exprAllowed: boolean,
+  inType: boolean,
+};