Allow Subinterpreters on WASI/Emscripten Builds

[ericsnowcurrently]

Currently using multiple interpreters on WASI and Emscripten builds is disallowed. This is reflected in various places, including configure, the test suite, and the docs. I don't recall why the feature was disallowed (maybe confusion about a relationship with threads?), but there shouldn't be any need to disallow them.

CC @brettcannon

[brettcannon]

Do the tests require processes for any reason? Otherwise I don't remember explicitly turning them off.

[freakboy3742]

@hoodmane can confirm, but IIRC, on Emscripten, the restriction is the lack of threads (or, at least, the decision to disable the limited thread support that is available on Emscripten because of security and compatibility concerns).