webstead: add Emscripten and musl copyright notices

webstead: add Emscripten and musl copyright notices

Programs compiled with Emscripten end up statically linked against the
Emscripten runtime, which includes musl libc.  So we need to comply with
their licences by including a copy of the licence and the relevant
copyright notices.  I think putting them on the webstead page in small
print is probably the best approach.

This might need updating each release to match the version of Emscripten
that I build the editor with.