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.

syntax/oil.vim

@@ -0,0 +1,7 @@
+if exists("b:current_syntax")
+  finish
+endif
+
+syn match oilId /^\/\d* / conceal
+
+let b:current_syntax = "oil"