Reuse completions from Bloop

[tgodzik]

Is your feature request related to a problem? Please describe.
In Bloop it's possible to get completions for test suites and main methods, since Bloop knows exactly which ones are available.

This can be also done via for example:
scala-cli bloop autocomplete --format fish --mode testsfqcn --project $project
scala-cli bloop autocomplete --format fish --mode mainsfqcn --project $project
which doesn't seem to work right now despite working in a normal Bloop project

Though project name would be the complicated one, but it could be appended automatically by Scala CLI

The examples for fish are here https://github.com/scalacenter/bloop/blob/main/etc/fish-completions

There are similar ones for bash and zsh next to it.

Describe the solution you'd like
Allow completions for tests suites and main classes

Additional context
I was digging into it when working on https://github.com/scalacenter/bloop/pull/2411/files