Add infrastructure to build and publish as wasm32

[lovell]

This is based on #3522

• Detects presence of emscripten at install time to re-configure node-gyp with relevant compiler/linker flags
• Adds CI job to build, test and generate prebuilt binaries via emsdk
• Package be published to npm as @img/sharp-wasm32 with a dependency on the excellent @emnapi/runtime
• Adds brief documentation (this feature is experimental)