feat: constrain_cursor option (closes #257)

doc/oil.txt

@@ -55,6 +55,9 @@ OPTIONS                                                              *oil-option
       -- Set to true to autosave buffers that are updated with LSP willRenameFiles
       -- Set to "unmodified" to only save unmodified buffers
       lsp_rename_autosave = false,
+      -- Constrain the cursor to the editable parts of the oil buffer
+      -- Set to `false` to disable, or "name" to keep it on the file names
+      constrain_cursor = "editable",
       -- Keymaps in oil buffer. Can be any value that `vim.keymap.set` accepts OR a table of keymap
       -- options with a `callback` (e.g. { callback = function() ... end, desc = "", mode = "n" })
       -- Additionally, if it is a string that matches "actions.<name>",