feat(experimental-utils): add literal types to global option (typescript-eslint#3634)

Author: dora1998

packages/experimental-utils/src/ts-eslint/Linter.ts

@@ -119,6 +119,8 @@ namespace Linter {
   export type RuleEntry = RuleLevel | RuleLevelAndOptions;
   export type RulesRecord = Partial<Record<string, RuleEntry>>;
 
+  export type GlobalVariableOption = 'readonly' | 'writable' | 'off' | boolean;
+
   // https://github.com/eslint/eslint/blob/v6.8.0/conf/config-schema.js
   interface BaseConfig {
     $schema?: string;
@@ -133,7 +135,7 @@ namespace Linter {
     /**
      * The global variable settings.
      */
-    globals?: { [name: string]: boolean };
+    globals?: { [name: string]: GlobalVariableOption };
     /**
      * The flag that disables directive comments.
      */