Drop ST3 support

[ayoub-benali]

With some refactoring and commands renaming Closes #49

• [X] drop st3 support
• [x] create an st3- tag (https://github.com/scalameta/metals-sublime/releases/tag/st3-v0.9.0)
• [x] make update repository https://github.com/sublimelsp/repository (https://github.com/sublimelsp/repository/pull/60)
• [X] refactor code
• [x] refactor logging

[ayoub-benali]

@rchl @rwols @predragnikolic I tried to refactor the code to follow more python conventions. Could you have a look ?