CMake: make it easier to build libCellML's WebAssembly

[agarny]

To build libCellML's WebAssembly currently requires doing quite a few things "by hand" while it would be much easier if all that someone had to do was to set something like -DBINDINGS_JAVASCRIPT=ON when configuring CMake (i.e. as we do with passing -DBINDINGS_PYTHON=ON when wanting to generate Python bindings) and then have something like ninja javascript_bindings to build everything that is needed to get libCellML's WebAssembly.