Problem: ext_map, filename_map, and resolve() are locked inside
override.lua as locals, making them inaccessible to any code path
that doesn't go through the devicons monkey-patch.
Solution: extract all resolution tables and logic into a new
nonicons.resolve module. Fix Gemfile/Jenkinsfile case bug (keys
were uppercase but resolve() lowercases input before lookup). Add
ft_map for vim filetypes that don't match extension or mapping keys.
