fix(ext/node): brand-check SecureContext._external accessor

[nathanwhitbot]

Summary

Node's tls.createSecureContext().context._external is a C++ accessor that returns the underlying external pointer; reading it on a non-context receiver hits an internal-slot check and throws TypeError. Deno had no _external accessor at all, so:

const pctx = tls.createSecureContext().context;
const cctx = { __proto__: pctx };
cctx._external;  // expected: TypeError; actual: silently undefined

This patch defines _external as a non-enumerable own getter on the context object, brand-checked against a private WeakSet of contexts the SecureContext constructor created. Reading it on the original context returns the context (we have no real external pointer); reading it via prototype-tampering throws TypeError("Illegal invocation"), matching Node's behavior.

Enables parallel/test-tls-external-accessor.js in the Node compat suite.

Test plan

• cargo test --test node_compat -- test-tls-external-accessor (was failing on main, passes here)
• cargo build --bin deno clean
• tools/format.js and tools/lint.js --js clean
• Sibling enabled TLS tests still pass: test-tls-basic-validations, test-tls-alert, test-tls-cert-regression, test-tls-honorcipherorder, test-tls-startcom-wosign-whitelist, test-tls-check-server-identity, test-tls-connect-simple.

[nathanwhitbot]

Targeted fix for the exact symptom — prototype-tampering reads of
_external on a non-pctx receiver now throw TypeError instead of
silently returning undefined.

The brand check via a module-scope WeakSet is the right shape:

• Constructor adds this.context to the set and installs the getter.
• Getter compares this (the property-access receiver) against the set —
  inherited objects, Reflect.get with a different receiver, etc. all fail
  the check.
• Returning this from the live path is fine because deno has no real
  external pointer and the upstream test only asserts "no throw" for that
  branch.

Worth verifying once at the implementation level:

• The descriptor uses configurable: true, which is more permissive than
  Node's C++ accessor — it could be deleted or redefined by user code.
  Probably fine since the test doesn't exercise it, but the safer match is
  configurable: false.
• __proto__: null on the descriptor is a nice touch — it dodges the
  Object.prototype pollution path that the recent child_process PR was
  fixing in the same neighborhood.
• No set is defined, so writes will throw in strict mode and silently
  no-op otherwise. Matches Node's accessor (no setter).

The WeakSet is module-scope and weakly held, so contexts won't leak after
their owning SecureContext is GC'd.

LGTM.