Provides two pieces of functionality:

1. A keepalive token that can be used to prevent the current POSIX
thread from terminating when it returns from its thread start
function, or from an invocation from the JS event loop.  When the last
keepalive token is destroyed (assuming Emscripten isn't keeping the
thread alive for other reasons) it will terminate as if by calling
`pthread_exit` and the webworker will be made available to other
threads

2. A `HasUnsettledInteropPromises` property that peeks
`_js_owned_object_table` to see if there are any promises created by
the interop subsystem that have not been fulfilled or rejected yet.

for mono_wasm_eventloop_has_unsettled_interop_promises we
can't use the _js_owned_object_table size because it contains
other interop objects, not just promises

And link to dotnet#85052 for more details