[Feature] Export utility to get latest supported compatibility_date from @cloudflare/vite-plugin

### Describe the solution

Export a utility from `@cloudflare/vite-plugin` that returns the latest `compatibility_date` supported by the installed version

Then frameworks can use the latest `compatibility_date` in generated configs

Idea came from @jamesopstad here: https://github.com/withastro/astro/pull/15047#discussion_r2630598446