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({})`.

.github/ISSUE_TEMPLATE/bug_report.yaml

@@ -70,8 +70,8 @@ body:
           spec = {
             {
               'barrettruth/canola.nvim',
-              init = function()
-                vim.g.oil = {}
+              config = function()
+                require('oil').setup({})
               end,
             },
           },