refactor: revert module namespace from canola back to oil

Problem: the canola rename creates unnecessary friction for users
migrating from stevearc/oil.nvim — every `require('oil')` call and
config reference must change.

Solution: revert all module paths, URL schemes, autocmd groups,
highlight groups, and filetype names back to `oil`. The repo stays
`canola.nvim` for identity; the code is a drop-in replacement.

lua/oil/pathutil.lua

@@ -0,0 +1,29 @@
+local M = {}
+
+---@param path string
+---@return string
+M.parent = function(path)
+  if path == '/' then
+    return '/'
+  elseif path == '' then
+    return ''
+  elseif vim.endswith(path, '/') then
+    return path:match('^(.*/)[^/]*/$') or ''
+  else
+    return path:match('^(.*/)[^/]*$') or ''
+  end
+end
+
+---@param path string
+---@return nil|string
+M.basename = function(path)
+  if path == '/' or path == '' then
+    return
+  elseif vim.endswith(path, '/') then
+    return path:match('^.*/([^/]*)/$')
+  else
+    return path:match('^.*/([^/]*)$')
+  end
+end
+
+return M