Implement callback completions

[Moosieus]

Callback completions should now expand to their full def-blocks, preceded with an impl tag.

In a module use-ing a behaviour, typing def | should result in the following completion options:

• def (define a function)
• def callback(args) module_label
• (remaining callbacks)
• Other def* macros

[Moosieus]

CI seems to be failing due to an unrelated issue:

 Could not fetch /builds/otp/ubuntu-22.04/builds.txt from any hex.pm mirror

In the meantime, I need to take a second pass at the callback completion and add additional tests. Presently def handle| doesn't overwrite the original def properly.

[scohen]

@Moosieus I re-ran the tests, these failures seem legit.

[Moosieus]

Keeping the sorting as-is for now since I've already pitched the overhaul in #643.