Add support for find_library in Emscripten

The find_library method can be used to find your own JavaScript libraries. The limitation is that they must have the file extension .js. Other library lookups will look up “native” libraries from the system like currently. A typical usage would look like this:

glue_lib = cc.find_library('gluefuncs.js',
                           dirs: meson.current_source_dir())
executable('prog', 'prog.c',
           dependencies: glue_lib)