feat: indent on enter after starting a focus block

[thorimur]

This PR adds the following to language-configuration.json to cause indentation after starting a focus block (·):

    "onEnterRules" : [
        {
            // Indent focus blocks
            "beforeText" : "^\\s*(·|\\.)\\s.*$",
            "action" : { "indent" : "indent" }
        }
    ]

Now, hitting enter after intro x yields

constructor
· intro x
  | <-- cursor ends up here

See Zulip.

---

Note: there are some cases where this will produce the incorrect indentation, e.g.

  · have : Foo := by
    | <-- cursor will be here

  · have : Foo := by
      | <-- cursor should be here

but handling EOL tokens like by is done in #329 .