Use $BASH_SOURCE to find location of emsdk_env.sh

$_ seems to be unreliable if the first command of my shell is to source the emsdk env script.

Bash source seems to be reliable though.
No, the #!/bin/bash was completely meaningless so I removed it (sourced files pay no attention to shebangs).

The comment is a note that because a user could be using any shell when they source this file, it's best not to assume bash features are available. Case in point, my first attempt at this patch used BASH_SOURCE exclusively, despite me usually being aware of the issue (my follow up patch corrected this to use $_ if bash source isn't available).