[Astro] Support Astro 5.x

[oliverlynch]

Currently, trying to update Astro to version 5+ fails due to a @bookshop/astro-engine peer dependency conflict.

npm error code ERESOLVE
npm error ERESOLVE unable to resolve dependency tree
npm error
npm error While resolving: [email protected]
npm error Found: [email protected]
npm error node_modules/astro
npm error   astro@"^5.1.3" from the root project
npm error
npm error Could not resolve dependency:
npm error peer astro@"^2.0.0 || ^3.0.0 || ^4.0.0" from @bookshop/[email protected]
npm error node_modules/@bookshop/astro-engine
npm error   @bookshop/astro-engine@"3.12.0" from the root project

Setting an override to version 5.1 seems to work fine, so hopefully all that needs changing is a version bump :)

Workaround:

  "overrides": {
    "@bookshop/astro-engine": {
      "astro": "^5.1.0"
    }
  }