`emsdk_dir` config variable

[Andrej730]

Hi! Migrated to pyodide xbuildenv install-emscripten from cloning emsdk myself. The minor thing I've noticed that typically after installation you might need to source its environment and for this currently I use

EMSDK_ROOT=$(pyodide config get pyodide_root)/../../emsdk
source ${EMSDK_ROOT}/emsdk_env.sh
which emcc

Since today and #321 it's also possible to do

EMSDK_ROOT=$(pyodide config get pyodide_root)/../../

But maybe there should be emsdk_dir, since it's probably the most useful one for accessing installed emscripten environment?

If it's okay, I can submit a PR similar to #321 for this

[agriyakhetarpal]

Oh yes, please do, @Andrej730! I think we can then merge the CHANGELOG entry for your PR and #321 into one sentence. Thanks for noticing that!