feat: add get_icon and get_icon_by_filetype API

Problem: plugins that don't use devicons have no way to get nonicons
glyphs for files. The only integration path is the devicons
monkey-patch via apply().

Solution: add get_icon(name, ext) and get_icon_by_filetype(ft) to
the public API in init.lua. Both use lazy require of the resolve
module and return nil on miss so callers decide fallback. Document
both functions in vimdoc and update the oil.nvim recipe.

lua/nonicons/init.lua

@@ -40,4 +40,18 @@ function M.apply()
   end
 end
 
+function M.get_icon(name, ext)
+  local key = require('nonicons.resolve').resolve_name(name, ext)
+  if key then
+    return M.get(key)
+  end
+end
+
+function M.get_icon_by_filetype(ft)
+  local key = require('nonicons.resolve').resolve_filetype(ft)
+  if key then
+    return M.get(key)
+  end
+end
+
 return M