modules/output: Allow to specify text to add as extra files (#343)

To enable some features (like adding tree-sitter queries) we need to add
files to specific directories in the runtime path (queries/lang/file.scm
for tree-sitter queries for example).

This commit adds support for specifying such files. You must be careful
to not have any collisions between `files` and `extraFiles`.

tests/test-sources/modules/extra-files.nix

@@ -0,0 +1,9 @@
+{
+  query = {
+    extraFiles = {
+      "queries/lua/injections.scm" = ''
+        ;; extends
+      '';
+    };
+  };
+}