Surface the embedded V8 version

There's no authoritative way to learn which V8 version a workerd release embeds, or which version production currently runs — yet that's exactly what determines the available JavaScript built-ins (it's the embedded V8 version, **not** `compatibility_date`, which only gates runtime API behavior). This was confirmed in [#517](https://github.com/cloudflare/workerd/issues/517):

> "We shipped V8 11.0 in late February, so these should have landed then, and AFAICT they are indeed supported today." — @kentonv

Since the V8 version is the only thing that answers "is this built-in available?" and it's undocumented, choosing `lib` in `tsconfig.json` is guesswork:

- `"lib": ["esnext"]` can type built-ins production V8 hasn't shipped → clean type-check, runtime `TypeError`.
- A conservative `"lib": ["es2022"]` silently hides features that *are* live.
- `workerd --version` prints only a date (`workerd 2026-05-07`) — no V8 version, and prod may run a newer V8 than any local binary.

The only workaround is folklore — grep V8-bump mentions from scattered threads, or file a "does workerd have X?" issue and wait for a maintainer. Not discoverable, doesn't scale. Please surface the V8 version in a discoverable form such as `workerd --version` and in every GitHub Release / changelog entry, making the local runtime self-describing and giving a release→V8 history — and state the V8 production currently runs somewhere stable, since prod can lead any pinned local binary.

Start by checking what `workerd --version` currently reports and how GitHub Releases and changelog entries identify releases. The issue asks for the embedded V8 version to be discoverable locally and per release, plus the V8 version currently used in production; done means all three sources are addressed.

Written by the indexing model from the issue text.