docs: misc 2.8 gaps (lib.node, task prefixes, watch, Cache keys, compat)

[bartlomieju]

Bundle of small 2.8 doc updates from the audit against the 2.8 blog post.

runtime/fundamentals/node.md

• Intro: noted that Node test-suite compat jumped from ~42% at the start of 2026 to >75% in Deno 2.8, with a link to node-test-viewer.deno.dev.
• "Including Node types": rewrote to lead with the 2.8 default (`lib.node` included automatically; `Buffer`, `NodeJS.*`, `process` resolve with no setup). Added the pinning escape hatch via `imports` for `@types/node`. Kept the `/// <reference types="npm:@types/node" />` form as the older fallback.

`runtime/reference/cli/task.md`

Added a `:::info Deno 2.8` admonition under "Task dependencies" noting that parallel-task output is prefixed with the task name (color-coded), and that prefixes stay attached through subprocesses.

`runtime/reference/cli/run.md`

• Cross-linked `--watch` to `deno test` / `deno serve` / `deno bench`.
• Added a `:::info Deno 2.8` admonition about the new `SIGTERM` + 500ms grace period when the watcher restarts the process, so `unload` listeners and `process.exit` hooks get to run.

(Per a quick post-audit pass, I dropped the `--watch-exclude` write-up — keeping the doc trim.)

`runtime/reference/web_platform_apis.md`

Added `CacheStorage::keys()` and `Cache::keys()` to the Cache API implemented-APIs list (the deviations section listed only 6 APIs before; these two land in 2.8).

Skipped from the original list

• TypeScript 6.0.3 version — pure implementation detail; no page references the TS version.
• `deno compile` progress bar — pure UX, nothing to configure.
• Web Crypto SHA3 / P-521 — algorithm support belongs in reference docs (auto-generated); no deviations text to update.
• `Deno.upgradeWebSocket` + `node:http` upgrade events — `Deno.upgradeWebSocket` is already covered; the Node compat side belongs in the auto-generated `node_apis.md`.