As per the discussion on slack.
Enable `comma-dangle` eslint rule + prettier config.
Good for two reasons:
1) (the main reason) It will create less noise in PRs:
```diff
 {
-  "foo": true
+  "foo": true,
+  "bar": false
 }
```
2) It makes reordering members easier because there is always a comma at the end.

----

Steps to create this PR:
1) add line in eslint config
2) add prettier config
3) run `yarn format`
4) ???
5) profit.

No other code was manually touched in the making of this PR.

* refactor(ts-estree): improve types in convertBodyExpressions

* refactor(ts-estree): change FirstTypeNode to TypePredicate

* test(ts-estree): add missing test cases

* chore(ts-estree): disable coverage for Debug.fail

* refactor(ts-estree): make sure that BindingElement is always converted

…) (#310)