lib/neovim-plugin: Add lua configuration scoped to the plugin

This commit adds a `plugins.<name>.luaConfig` section controlling the
plugin specific configuration.

The section contains the internal `init` option, containing the plugin's
initialization code.

It also contains the public `pre` and `post` options, that allow to add
code before & after the `init` section

Finally, it contains the `final` option, being the concatenation of the
three previous options.

tests/test-sources/plugins/lua-config.nix

@@ -0,0 +1,26 @@
+{
+  lua-config-pre-post = {
+    extraConfigLuaPre = ''
+      list = {}
+    '';
+    plugins.cmp = {
+      enable = true;
+      luaConfig = {
+        pre = ''
+          table.insert(list, "pre")
+        '';
+        content = ''
+          table.insert(list, "init")
+        '';
+        post = ''
+          table.insert(list, "post")
+        '';
+      };
+    };
+    extraConfigLuaPost = ''
+      if not vim.deep_equal(list, {"pre", "init", "post"}) then
+        vim.print("Unexpected list: " .. vim.inspect(list))
+      end
+    '';
+  };
+}