Intent to prototype: WebAssembly extended constants

[Ryan Hunt]

Summary:
Adds new instructions to the list of constant instructions that can be
used in constant expressions. Constant expressions are used to initialize
WebAssembly globals, and to determine the offset at which to apply
data/element segments to memories/tables respectively. This is intended to
improve support for dynamic linking of WebAssembly modules. See [1] for
more background.
Bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=1706124
Standard:
https://github.com/WebAssembly/extended-const
Platform coverage:
All platforms that support WebAssembly. Limited to Firefox Nightly only.
Preference:
javascript.options.wasm_extended_const
DevTools bug:
No bug, I don't believe any work is required here.
Other browsers:
As far as I'm aware, no other browsers support this or sent public intents.
web-platform-tests:
The proposal repository contains tests that we will import. We will also
write our own tests internal to SpiderMonkey.

Thanks,
Ryan

[1] https://github.com/WebAssembly/extended-const/blob/master/proposals/extended-const/Overview.md