Expose --init functionality in a tsserver command

[mhegazy]

We now have --init support to generating a default tsconfig.json file (see #4037). Editors would need to expose a functionality that uses this, possibly passing a set of additional defaults based on their scenario. we need to expose this as a Language Service API as well as a tsserver command. see @jrieken's comment in #4037 (comment)

[mhegazy]

//CC: @tinganho