optionally build a shared library

[brandonros]

I'm not very good at CMake so let me know how this can be better.

[brandonros]

I'm not quite sure what the "on library load" function should be, let alone the named exports we need to now export. main() would need to be converted, and we'd need a way to access "should-be-globals" like TJSRuntime *qrt and JSContext *ctx

[brandonros]

$ nm build/libtjs.dylib | grep TJS_GetJSContext
000000000000fb84 T _TJS_GetJSContext
$ nm build/libtjs.dylib | grep TJS_GetRuntime
000000000000fb8c T _TJS_GetRuntime

Not bad.

[saghul]

Good start! I think we bend some macro helper to export symbols on the library.

Alternatively, the binary itself could and be like a library. Node does this for example.