[Enhancement] dir: Optionally fallback to repo name with `truncate_to_package_name`

[wrboyce]

Adds three new settings: P9K_DIR_SHORTEN_GIT_FALLBACK, P9K_DIR_SHORTEN_GIT_FALLBACK_USE_REMOTE, and P9K_DIR_SHORTEN_GIT_FALLBACK_REMOTE_NAME for the truncate_to_package_name path shortening strategy.

If GIT_FALLBACK is non-zero and a package.json/composer.json does not exist then the repo directory name will be used as the "root" directory in the prompt path label. If USE_REMOTE is non-zero, then the remote of name REMOTE_NAME (defaulting to "origin") will be used in-place of the local dir name.