add short CLI option -P for --project

[cjdoris]

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.

[cjdoris]

Because this option is now quite long, I have indented the help text a little:

 -P, --project[={<dir>|@temp|@.|@script[<rel>]}]  Set <dir> as the active project/environment.
                                               Or, create a temporary environment with `@temp`
                                               The default @. option will search through parent
                                               directories until a Project.toml or JuliaProject.toml
                                               file is found. @script is similar, but searches up
                                               from the programfile or a path relative to
                                               programfile.
 -J, --sysimage <file>                         Start up with the given system image file
 -H, --home <dir>                              Set location of `julia` executable

Is this OK or would you rather realigning like this?

 -P, --project[={<dir>|@temp|@.|@script[<rel>]}]  Set <dir> as the active project/environment.
                                                  Or, create a temporary environment with `@temp`
                                                  The default @. option will search through parent
                                                  directories until a Project.toml or JuliaProject.toml
                                                  file is found. @script is similar, but searches up
                                                  from the programfile or a path relative to
                                                  programfile.
 -J, --sysimage <file>                            Start up with the given system image file
 -H, --home <dir>                                 Set location of `julia` executable

Or putting it all onto the next line like this?

 -P, --project[={<dir>|@temp|@.|@script[<rel>]}]
                                               Set <dir> as the active project/environment.
                                               Or, create a temporary environment with `@temp`
                                               The default @. option will search through parent
                                               directories until a Project.toml or JuliaProject.toml
                                               file is found. @script is similar, but searches up
                                               from the programfile or a path relative to
                                               programfile.
 -J, --sysimage <file>                         Start up with the given system image file
 -H, --home <dir>                              Set location of `julia` executable

[cjdoris]

The test failures look unrelated to this.

[LilithHafner]

Seems quite reasonable to me. Tagging triage as this expands the public API.