This takes airlift's 'suggest' command and reuses it as a different
command name, 'completion'. This gives us in-built CLI completions which
are useful in the repo-level container's docker-entrypoint.sh. This
previously parsed Java files for conventional usage of Command
annotations, which is potentially buggy. The new implementation relies
only on CLI to provide command completion suggestions.

As part of this, we can prepare for bash completion scripts which can be
added to our homebrew formula.  The new completion command will also
complete on command options, for example:

cli completion generate

This will provide all short and long form switches available to the
generate command.

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