metro v0.83.6 release notes

This release is on the 0.83.x branch

[Feature]: Add keepalive "heartbeat" message to Fast Refresh connections (https://github.com/facebook/metro/pull/1685 by @robhogan)
[Feature]: Add changeId metadata on HMRClient update-done messages (https://github.com/facebook/metro/commit/1585e444284f by @huntie)
[Fix]: Include user-defined Babel config in transformer cache key to ensure correctness (https://github.com/facebook/metro/pull/1638 by @evanbacon)
[Fix]: Detection of contents of directories moved or cloned into a watched location (https://github.com/facebook/metro/pull/1660 by @robhogan)
[Fix]: Correctly detect contents as removed when a watched directory is moved, on macOS without Watchman (https://github.com/facebook/metro/commit/c9e9fe64a77c by @robhogan)
[Performance]: Optimize PackageCache: use Map/Set, cache null results, eliminate per-hit allocations (https://github.com/facebook/metro/commit/877b64ea5967 by @robhogan)
[Types]: Restore type exports from metro that were previously TypeScript-specific (https://github.com/facebook/metro/pull/1672 by @robhogan)

NOTE: Experimental features are not covered by semver and can change at any time.

[Experimental]: Add config.unstable_fileMapPlugins (https://github.com/facebook/metro/commit/567aa8b62b95 by @robhogan)