[EH] Fuzz calls from JS by calling wasm exports, sometimes catching (#7067)

This adds two new imports to fuzzer modules:

* call-export, which gets an export index and calls it.
* call-export-catch, which does the call in a try-catch, swallowing
  any error, and returning 1 if it saw an error.

The former gives us calls back into the wasm, possibly making various
trips between wasm and JS in interesting ways. The latter adds a
try-catch which helps fuzz wasm EH.

We do these calls using a wasm export index, i.e., the index in
the list of exports. This is simple, but it does have the downside that
it makes executing the wasm sensitive to changes in exports (e.g.
wasm-merge adds more), which requires some handling in the fuzzer.

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