docs: add structured clone and transferable API reference

[bartlomieju]

Summary

• Adds tables documenting which Web API types are serializable (support structuredClone() / postMessage()) and which are transferable
• Updated both the API reference page (api/web/index.md) and the runtime reference page (runtime/reference/web_platform_apis.md)
• Motivated by the addition of Blob/File structured clone support in feat(ext/web): support structured clone for Blob and File deno#33827

Serializable types documented

Primitives, Array, Object, Map, Set, Date, RegExp, ArrayBuffer, TypedArray, DataView, Error types, Blob, File, DOMException, CryptoKey

Transferable types documented

ArrayBuffer, MessagePort, ReadableStream, WritableStream, TransformStream

Test plan

• Verify tables render correctly on the docs site
• Verify links resolve properly

[lunadogbot]

Two blockers:

1. deno fmt --check fails on both new tables — column alignment. Running deno fmt api/web/index.md runtime/reference/web_platform_apis.md fixes it.

2. The "Clone a Blob" sample doesn't run on current stable. denoland/deno#33827 merged 2026-05-07, but the most recent release is 2.7.14 (2026-04-28), so on stable structuredClone(new Blob([...])) returns {} and the example throws TypeError: cloned.text is not a function. Same for the File row. Either gate this on the next release (note "Requires Deno 2.8+" near Blob/File) or hold the PR until 2.8 ships.

• nit: runtime/reference/web_platform_apis.md has last_modified: 2025-05-09 — bump it on substantive additions.
• nit: the transfer example uses a worker symbol that isn't defined in the snippet. const ch = new MessageChannel(); ch.port1.postMessage(buffer, [buffer]); is self-contained and demonstrates the same point.