[Feature Request] Add `shards targets` to show all defined targets in shard.yml.

[zw963]

I probably need run some target to create index before build the finally binary, I need a easy way to check if there exists this target, check this use case, I have to shards run index at first to generate assets files, then shards build --release --static crystal_china to baked those assets into binary, but I don't hard code the index as the target name.

I haven't read the source code yet, but I think this feature is easy to implement. would you consider accepting this feature as a PR?

[zw963]

BTW: output target info, is useful for figure out what is the binary name after built success. e.g. the target crystal_china, should be ./bin/crystal_china.