Running GC based on memory limits

[guybedford]

At Fastly we have a pre-defined maximum memory usage for Wasm services.

It could be useful to be able to hard-code this limit into the runtime, so that the runtime knows when memory is running low in order to run the GC, and run more regular and aggressive GCs as the service memory limit is approached.

Might be a great feature to have in StarlingMonkey itself.