Streamline settings files

**WIP: Not ready for merging**

We'll now have this in the ST UI:

a) Preferences: Dart - Default
b) Preferences: Dart - User
c) Preferences: Dart - File Type (User)

(b) and (c) are user-editable.
(a) is read-only and not meant to be edited by users; only
    consulted for documentation, and used by this plugin to
    provide default values for settings.

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**TODO**

- [x] edit docs
- [x] test on Linux
- [x] test on Mac