Add support for nvim-dap and nvim-jdtls

[dradtke]

What

Add the nvim-dap and nvim-jdtls plugins and configure them.

Why

These plugins provide better support for editing and debugging Java code than what we currently have.

One caveat is that it may conflict slightly with #201, since the picker is likely not aware of nvim-jdtls's implementation of code actions, so some additional work may be needed in order to get these two PRs to play nicely.

Checklist

• Send RFC email to Braintree developers if change may be impactful. Please include a link to this PR so that discussion about the pros and cons of the change remains linked to the proposed changes. Recent examples include: addition of linter and completer, no longer removing end-of-line whitespace on save, changing to fzf for file lookup.