Adds a simple ESLint plugin for custom environments.
The plugin has a single exported value named `globals`, which is an object with
keys for all globally available self-hosted identifiers. All self-hosted values
are read-only, so we set all properties of `globals` to `"readonly"`.
BytecodeEmitter special identifiers are added to the `.eslintrc.js` file,
because that keeps them closer to the SpiderMonkey source tree when compared
to "tools/lint/eslint/eslint-plugin-spidermonkey-js".
Also see:
- tools/lint/eslint/eslint-plugin-mozilla/lib/environments/
- https://eslint.org/docs/latest/user-guide/configuring/language-options
Differential Revision: https://phabricator.services.mozilla.com/D153337
96daf134a0