Refactor LanguageHandler to AbstractPlugin

[rwols]

The ST4 version of LSP uses a new plugin mechanism, see the AbstractPlugin class. The LanguageHandler class will be removed in a future version: sublimelsp/LSP#1936

[rwols]

The above mentioned pull request is now merged, which makes this package not work in ST4.

[slrtbtfs]

Thanks for notifying us!

To be fair, this Plugin was already broken in ST4 before your upstream change.

@nevill Are you still interested in working on this Plugin to eventually make it work again?