Add Language Server Protocol support #140
Description
LSP is a fairly common protocol for handling Intellisense-like stuff in a variety of editors. Trying to use bfg9000 scripts in LSP-enabled editors can result in a bad experience, since the bfg9000 builtins aren't recognized by the Python LSP backend. You end up getting a bunch of "undefined symbol" errors while editing, which is annoying and can make it harder to find the real errors.
If we added our own LSP backend, this would make life a lot nicer for people editing scripts in an LSP-enabled editor.