feat: allow plugins to register additional unhandledRejection filters

[Pavelgq]

What's done?

Context:

• Any unhandled rejection in a worker terminates the whole run (fix: handle unhandled rejections #744) — a safety net for tests missing an await
• shouldIgnoreUnhandledRejection already carves out an exception for known Puppeteer/CDP noise, but it's a hardcoded, closed list
• Ran into a case where a plugin (not an actual test) triggers an unrelated async rejection while just reading a file for its own config, before any test even starts — same blanket policy kills the whole run for that too, with no way to tell Testplane "this one's fine"

This adds registerUnhandledRejectionFilter(filter) (exposed via testplane/unstable) so a plugin or integration can register its own predicate for cases like that, instead of it needing to get hardcoded into core the way Puppeteer's was:

import { registerUnhandledRejectionFilter } from "testplane/unstable";

const unregister = registerUnhandledRejectionFilter(err => isKnownSafeToIgnore(err));

A filter can only ever make something get ignored, never the reverse, and a filter that throws is just treated as "no match" and logged, so a broken filter can't hide a real error. The built-in Puppeteer allowlist still takes priority. Default behavior is unchanged for everyone who doesn't use this.

How I tested?

Unit tests covering filter registration/unregistration, multiple filters, a throwing filter not breaking anything else, and the built-in Puppeteer allowlist still taking priority. Full existing suite is still green.