Add ability to navigate to 'typedef' and 'class' symbols

This PR adds a `Symbol List.tmPreferences` file to define symbol transformations in it (usable with *Ctrl+R* in Sublime Text).

It also modifies the `Dart.tmLanguage` file to define scopes needed for symbols to work. I've also deleted entries that are ignored by ST.

#### About the generation of `Dart.tmLanguage`

I haven't edited the file directly. Instead, I've used [AAAPackageDev](https://sublime.wbond.net/packages/AAAPackageDev). With it, I follow the following workflow:

* Translate `Dart.tmLanguage` to `Dart.YAML-tmLanguage`
* Edit `Dart.YAML-tmLanguage`
* Translate `Dart.YAML-tmLanguage` to `Dart.tmLanguage`

(After this, I've removed the `Dart.YAML-tmLanguage` file to avoid confusion.)

There's a `.JSON-tmLanguage` in this repo that does not appear to be JSON at all. I don't know what tool's used to convert that file to `Dart.tmLanguage`, so that's why I've used the process above. I honestly don't know what's best, but I think we should find a standard process to edit the `.tmLanguage` file. For example, now the `.JSON-tmLanguage` in the repo is out of sync with this version.