Add 'Browse API Reference' command to Command Palette

Adds:

- `.sublime-commands` file with a 'Browse API Reference' command
-  <kbd>Ctrl+.</kbd>, <kbd>Ctrl+.</kbd> to open the Command Palette and see 'Dart: ' commands
- .py file so support 'Browse API Reference' command

TODO: If merged, documentation about available commands in the Command Palette.

(BTW, having to edit the docs online is very inconvenient. Could  at least the wiki be public to contributors?)