chore(astro): Add @sentry/astro package

[Lms24]

This PR adds the boilerplate files for the new @sentry/astro package. Amongst the usual files, some noteworthy things include:

• Bumped the Volta Node version from 16 to 18 for the entire monorepo. I think this should be fine, given that this has no effect on the Node versions we test with in CI.
• Using Vitest instead of Jest, like we do in SvelteKit. I think this is fine and should only lead to faster test execution times. It works well in SvelteKit
• We don't know exactly yet how we're going to bundle the package so for now the rollup config is just a placeholder but things might still change here.

ref #9182

[HazAT]

Should we already add the Astro keywords?
https://docs.astro.build/en/reference/publish-to-npm/#keywords

[Lms24]

I initially wanted to wait until we have something usable but then again, we'll iterate fast, so we can add it now, too.

[Lms24]

I opted for a separate PR because I had to configure the package as public to get our e2e tests working. We can merge whenever we feel comfortable with actually publishing the package.
#9193