fix: ftl new uses project root as dir

[matt2e]

fixes #3012

[alecthomas]

This doesn't seem like the right solution to either of these issues to me?

For the first, ftl commands that rely on a project should immediately error with a useful message if they're used outside of a project.

For the latter...that sounds like a bug that should be fixed, not worked around?

[matt2e]

Had another go, this time the fix is that ftl new uses the project root as it's plugin dir. This means we don't create and leave around an extra folder when we run ftl new or ftl new --help as we can assume {root}/.ftl exists or will exist anyway

The error message should also explain to the user what they need to do to make it work as well (ftl init)