Use reflex-dom-core in library

[hamishmack]

The reflex-dom package introduces a dependency on a default jsaddle runner package. Using reflex-dom-core instead lets the library user choose a different runner.

Here is a similar change we made to the reflex-dom-contrib library https://github.com/reflex-frp/reflex-dom-contrib/pull/36/files

The test can still use reflex-dom.

[imalsogreg]

This is closed by #3