Move Nix files into a nix directory

Move everything from `config` to the root level.

nvim/after/ftplugin/rst.lua

@@ -0,0 +1,13 @@
+local opt = vim.opt_local
+
+opt.spell = true
+opt.wrap = true
+
+local cmp = require "cmp"
+local sources = cmp.get_config().sources
+
+-- TODO: confirm these aren't aleady in the list of sources to avoid duplicate suggestions.
+table.insert(sources, { name = "buffer" })
+table.insert(sources, { name = "path" })
+
+cmp.setup.buffer { sources = sources }