JS shell on WASI: add basic Wizer integration for standalone testing.

[cfallin]

This patch allows the use of a JS shell binary, compiled to wasm32-wasi (WASI ABI on WebAssembly), to make use of the Wizer snapshotting tool to embed JavaScript source with SpiderMonkey to run in a standalone Wasm module.

If enabled with --enable-js-shell-wizer, the shell no longer provides an interactive REPL; instead, when invoked during the "initialization" phase of Wizer, it reads JS source on standard input, then saves JS state (the global, the context, etc) for later. When the execution is snapshotted and the snapshot is later executed as an ordinary WASI command, the resume entry-point invokes a JS function named main in the global scope, if any.

An example of how to use this follows:

$ echo "function main() { print('hello world'); }" | \
  wizer --allow-wasi --wasm-bulk-memory=true -r _start=wizer.resume obj-release/dist/bin/js \
  -o snapshotted.wasm

$ wasmtime snapshotted.wasm
hello world
$

Note that embedders of SpiderMonkey on Wasm can and do add their own Wizer integration; the purpose of this PR is to add a standalone mode that does not require building SpiderMonkey within another context in order to evaluate features (such as AOT compilation) that require snapshotting as a prerequisite. Followup PRs will include test results derived with this simple Wizer integration.

[JakeChampion]

Approved but I think we might need to include a copy of the Wizer license within third_party/wizer