[RFC] Add mixed js-ts config

[ShGKme]

There are some rules that are problematic in mixed JS-TS repositories, which is quite a wide case in our ecosystem.

While we can define different rules for .js and .ts files, we cannot do it for .vue files (they can be both JS and TS).

Currently we can:

• Make TS config more friendly for mixed repos (less strict)
• Force developers to override the config

Instead, we can provide a config for shared environments:

• TS config is still strict
• Developers don't need to make the same overrides

[ShGKme]

• Alternative: [RFC] Add strict config(s) for optioned rules #1005