[babylon] Refactor mixin plugins handling & validation (#7999)

packages/babel-parser/src/parser/base.js

@@ -4,12 +4,13 @@ import type { Options } from "../options";
 import { reservedWords } from "../util/identifier";
 
 import type State from "../tokenizer/state";
+import type { PluginsMap } from "./index";
 
 export default class BaseParser {
   // Properties set by constructor in index.js
   options: Options;
   inModule: boolean;
-  plugins: { [key: string]: boolean };
+  plugins: PluginsMap;
   filename: ?string;
   sawUnambiguousESM: boolean = false;