Add `.prettierignore` and the `.prettierrc` config file. Prettier is currently targeting the `JSON`, `Markdown` and `YAML` files in the mruby repo.

Additional file types can be targetted with prettier via additional community plugins.

https://prettier.io/docs/en/

You can see all the languages that are supported on the homepage:

https://prettier.io/

This PR references a few other issues and PRs

We did have prettier running with pre-commit before that method was removed from prettier.

https://github.com/mruby/mruby/pull/6292

https://github.com/mruby/mruby/issues/6291

https://github.com/mruby/mruby/issues/6138

https://github.com/mruby/mruby/pull/5490

Perhaps we could add more npm software to pre-commit in future using new the `local` repo approach.

https://pre-commit.com/#repository-local-hooks

The best part is that it runs with pre-commit on our local machines on `git commit` and also when running `pre-commit run --all-files` or for example when running `make check`.

There were also some minor fixes / style changes in the pre-commit config to standardize casing.

https://www.npmjs.com/package/prettier?activeTab=readme