refactor(text): extract text helper to rrvsh/text.nix

This commit adds the rrvsh/text.nix flake to manage text generation.
It also removes the old text generation helpers module and updates the
README and flake.nix files to use the new flake.

modules/flake-parts/text.nix

@@ -0,0 +1,4 @@
+{ inputs, ... }:
+{
+  imports = [ inputs.text.flakeModules.default ];
+}