fix: don't destroy combined statuslines on uninstall (#479)

uninstall deleted the whole statusLine key whenever the command contained
'ponytail-statusline'. For a user-combined statusline (e.g. caveman && ponytail)
that destroyed the other plugin's statusline too. Only remove the statusLine
when ponytail owns the whole command; if it's combined with other segments,
leave it and tell the user to strip ponytail's part by hand. Never leaves a
malformed husk. Adds a regression test for the combined case.

Closes #374

Co-authored-by: nanaubusiness <195150264+nanaubusiness@users.noreply.github.com>