[build2] Listing available targets

[Per Edin]

Hi!

Is it possible to list available targets in the current directory
similar to what is usually output when you run "make help"?

I sometimes find it difficult to figure out what to type to just
compile a single .cxx-file.

Also, trying to build a single unit test like `b exe{foo.test}` fails
with "unknown target", although `b` does print that target when
building everything ("ld exe{foo.test}"). Being able to list targets
would probably fix this problem too, unless it's a bug.

Kind regards
Per Edin