fix(project): Add polling fallback for file watching in containers - #1511

In many container setups (Docker over overlayfs, bind mounts, network
filesystems) Linux inotify either does not deliver events or the native
@parcel/watcher thread dies on EINTR: a SIGCHLD from a child process
interrupts the blocking poll(), which the watcher treats as fatal (see
parcel-bundler/watcher issue #253). In both cases subscribe() resolves and
then goes silent, with no error to catch. The incremental build derives
"what changed" solely from watcher events, so a silent watcher breaks
rebuilds and live reload entirely.

Parcel offers no polling subscription (its brute-force backend is
query-only), so add one in a new module, pollingWatcher.js, that all three
watcher consumers call instead of @parcel/watcher directly. It exposes a
subscribe() matching @parcel/watcher's exact contract. The backend is chosen
once per process and memoized: UI5_WATCH_MODE=polling|native forces it,
otherwise a startup test subscribes natively to a temp dir on the same
filesystem as the real watch, writes a file, and falls back to polling if no
event arrives within 1500 ms.

The polling path walks the tree and diffs an mtimeMs+size snapshot every
250 ms (rescheduling itself after each walk so a slow crawl cannot overlap
the next), emitting the same {type, path} events the native backend would.
Errors flow through the callback so each consumer's existing recovery path
fires unchanged. The 250 ms interval stays below WATCHER_BURST_SETTLE_MS
(550 ms) so downstream event batching still holds. Ignore globs reuse
micromatch (already a direct dependency) and prune ignored directories so
node_modules is never crawled.

WatchHandler, ProjectDefinitionWatcher, and projectGraphSettleWatcher swap
their parcelWatcher.subscribe call for the new module; callback bodies,
ignore globs, and settle windows are untouched. UI5_WATCH_POLL_INTERVAL
overrides the interval (clamped).

A mid-run EINTR death after a passing startup test is not covered; force
UI5_WATCH_MODE=polling in that case.

Fixes: #1479