ci: properly ignore generated TS directory (#9383)

Only ignoring these in the pre-commit check is not enough as that only
affects CI. Instead, by adding these directories to the
`.prettierignore` file, they are also ignored when prettier is invoked
to format files.

.github/pre-commit-config.yaml

@@ -10,7 +10,6 @@ repos:
       - id: prettier
         name: Prettier
         entry: prettier --check --ignore-unknown .
-        exclude: (^rust/gui-client/src-frontend/generated/)
         language: system
         pass_filenames: false