Add emscripten-wasm32 and wasi-wasm32 platforms to known platforms

[wolfv]

This adds two new platforms to conda's list of known platforms, to unblock some of the experimental work happening on emscripten-forge. Names are derived from what CPython and Rust have chosen to use.

[wolfv]

Thanks @jezdez for adding the news :)

[philippjfr]

Yay, glad to see we were able to act quickly on this!

[dholth]

I checked that conda-index is getting this list from the correct place, and should be able to index these platforms.

>>> conda_index.utils.DEFAULT_SUBDIRS
{'linux-armv7l', 'linux-32', 'linux-s390x', 'linux-armv6l', 'win-32', 'win-64', 'osx-arm64', 'zos-z', 'win-arm64', 'linux-ppc64', 'linux-ppc64le', 'linux-aarch64', 'osx-64', 'wasi-wasm32', 'freebsd-64', 'linux-riscv64', 'emscripten-wasm32', 'noarch', 'linux-64'}