fix(ftp): give oil-ftps:// its own adapter name to prevent scheme clobbering

Problem: both oil-ftp:// and oil-ftps:// mapped to the adapter name
'ftp', so config.adapter_to_scheme['ftp'] was set to whichever scheme
pairs() iterated last — non-deterministic. init.lua uses
adapter_to_scheme[adapter.name] to reconstruct the parent URL, so
roughly half the time it injected 'oil-ftps://' into ftp:// buffer
navigation, causing the ssl-reqd error on '-' press.

Solution: register oil-ftps:// under adapter name 'ftps' via a
one-line shim that returns require('oil.adapters.ftp'). Now
adapter_to_scheme['ftp'] = 'oil-ftp://' and
adapter_to_scheme['ftps'] = 'oil-ftps://' are both stable.

lua/oil/adapters/ftps.lua

@@ -0,0 +1 @@
+return require('oil.adapters.ftp')