Cylc install: Shortcut to `--flow-name=basename`

[wxtim]

Describe exactly what you would like to see in an upcoming release

In the original Cylc install proposals it was suggested that

cylc install a/b/c

should install into ~/cylc-run/c. At some point this behaviour got changed to installing ~/cylc-run/a/b/c and the consensus in #4643 seems to be to keep this behaviour.

For user convenience it my be handy to have a shortcut to cylc install <path> --flow-name=$(basename <path>). It could be called cylc install --flat

Additional context

Follow up to https://github.com/cylc/cylc-flow/issues/4643

Low priority because:

Adding this feature wouldn't alter existing user work patterns but add an extra option.

Pull requests welcome!