src: register external references for source code

Currently we use external strings for internalized builtin source code. However when a snapshot is taken, any external string whose resource is not registered is flattened into a SeqString (see ref). The result is that any module source code stored in the snapshot does not use external strings after deserialization. This patch registers an external string resource for each internalized builtin's source. The savings are substantial: ~1.9 MB of heap memory per isolate, or ~44% of an otherwise empty isolate's heap usage:

$ node --expose-gc -p 'gc(),process.memoryUsage().heapUsed'
4190968
$ ./node --expose-gc -p 'gc(),process.memoryUsage().heapUsed'
2327536

The savings can be even higher for user snapshots which may include more internal modules.

Doing this with the existing UnionBytes abstraction was tricky, because UnionBytes only creates an external string resource when ToStringChecked is called. We change UnionBytes to no longer own the data, and shift ownership of the data (in the case where the data is not static) to a new external resource class. We can then register any resource whose data is static, since we know those are precisely the internalized builtins.

Refs: https://github.com/v8/v8/blob/d2c8fbe9ccd1a6ce5591bb7dd319c3c00d6bf489/src/snapshot/serializer.cc#L633