add short CLI option -P for --project (#59867)

For me `--project` is by far the most-used CLI option but it doesn't
have a short version. This PR adds the alias `-P` so now you can do
`julia -P @foo`, saving 7 precious keystrokes over `julia
--project=@foo`.

Note that as a short option, the argument is required, so you cannot do
`julia -P`, you must use `julia -P @.`. This is the same limitation as
`-O` whose argument is optional when used in its long form.

This also updates the relevant bits of the `--help` string, docs and man
page; and adds some tests by duplicating ones using `--project`.

---------

Co-authored-by: Christopher Doris <github.com/cjdoris>