refactor: revert canola namespace to oil and remove vim.g config (#120)

* 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.

* refactor: remove `vim.g.oil` declarative config

Problem: the `vim.g.oil` configuration path was added prematurely.
It adds a second config entrypoint before the plugin has stabilized
enough to justify it.

Solution: remove `vim.g.oil` support from `plugin/oil.lua`,
`config.setup()`, docs, and tests. Users configure via
`require("oil").setup({})`.

lua/oil/constants.lua

@@ -0,0 +1,13 @@
+local M = {}
+
+---Store entries as a list-like table for maximum space efficiency and retrieval speed.
+---We use the constants below to index into the table.
+---@alias oil.InternalEntry {[1]: integer, [2]: string, [3]: oil.EntryType, [4]: nil|table}
+
+-- Indexes into oil.InternalEntry
+M.FIELD_ID = 1
+M.FIELD_NAME = 2
+M.FIELD_TYPE = 3
+M.FIELD_META = 4
+
+return M