Pop execution context off the stack after InitializeHostDefinedRealm

[TimothyGu]

InitializeHostDefinedRealm is defined by ECMAScript to push the newly created execution context to the stack. This is especially visible after #3314. However, in the four callsites of InitializeHostDefinedRealm in the spec, none of them actually pop the new execution context off the stack after they are done, which can lead to many issues. We probably want to create an algorithm that creates the realm and execution context and pops it off the stack for us, and use that algorithm whenever we call InitializeHostDefinedRealm.

One question is if we want to use clean up after running script instead of manual popping. Probably not though, since we don't want most algorithms that create a new realm to run microtasks as a side effect.

/cc @domenic