[wip] add doit

[bollwyvl]

This is an exploration of refining the developer experience with a file-based build automation system.

In addition to the fine-grained tasks, there are a few common ones:

• LAB binder runs this. sets up the js/python for a working dev lab
• LINT format/lints everything
• ALL is heavy, but should be useful when switching branches
• WATCH is okay, but interleaving the tsc, webpack and lab output is pretty busy...
• RELEASE is useful for just getting to some files
  • could be extended to actually do the release, but meh...

(if we want to keep doit, we can generate/customize the above graph in #177 with doit-graph)

References

• reduce number of top-level files #180 https://github.com/krassowski/jupyterlab-lsp/pull/180#issuecomment-578440914

Code changes

• adds a make-like system with doit
• use in binder
• use in CI

User-facing changes

None

Backwards-incompatible changes

• lint.py is removed in favor of the more granular approach
• some output paths were changed to make them more predictable

Chores

• linted
• tested
• documented
• changelog entry

[bollwyvl]

I'd still like to see a top-level automation tool that wasn't (entirely) hand-cobbled scripts, but this has bitrotted fairly badly at this point.