Add @babel/helper-validator-identifier (#11289)

* refactor: replace esutils.keywords.isIdentifierNameES6 by helper-validator-identifier

* refactor: replace esutils isReservedWordES6 by isKeyword || isReservedWord

* address review comments

* chore: specify both “main” and “exports”

* build helper-validator-identifier before babel-types

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

@@ -163,6 +163,8 @@ export const types: { [name: string]: TokenType } = {
   }),
 
   // Keywords
+  // Don't forget to update packages/babel-helper-validator-identifier/src/keyword.js
+  // when new keywords are added
   _break: createKeyword("break"),
   _case: createKeyword("case", { beforeExpr }),
   _catch: createKeyword("catch"),