Remove `jsSource` and `jsCommonJS`; not part of JavaScript
Add `jsIdentifier`
Add `jsFuncCall`
Remove `-` from valid characters for `jsLabel`
Link `jsLabel` to `Identifier` highlight group
Add `jsNoise` so users can subdue noisy syntax clutter
Update `jsType` and `jsOperator`

- Move `void` and `yield` from `jsType` to `jsOperator`

- Remove `undefined` from `jsType`

- Add `jsUndefined` keyword; link to `Constant` highlight group

- Add logical / comparison / assignment operators to `jsOperator`
Add `jsNan` keyword; link to `Constant` highlight group
Refactor `jsFunction`/`jsFuncName`/`jsFuncBlock`/`jsOpAssign`

- Fix messy nested logic regarding `b:javasript_fold` by moving some
  stuff out of if statements

- Change `jsFuncName` to match only the function name
Change highlight links to be more semantically correct

- `jsReturn` links to `Statement`

- `jsFunction` links to `Type`

- `jsFuncName` links to `Function`

- `jsThis` links to `Special`

- `jsNull` links to `Constant`