PSA: emsdk switch to `main` as primary branch name

[Sam Clegg]

See https://github.com/emscripten-core/emsdk/issues/805

As per the plan laid in the issue above we will be keeping the master branch around in this case so that we have some way to re-direct folks who use emsdk to the new branch.

Once we make the switch, the next time you run an `emsdk` command from the `master` branch it will instruct you how to switch to using `main` instead.

cheers,

sam

[Sam Clegg]

The switch has now been made.  `master` continues to exist for now and user's who use this (emsdk) branch should see a warning with instructions on how to switch to `main`.