canvas (public) interpreter functions are only loaded when in canvas app

[ppisljar]

public canvas functions are now registered in components/app/index.js which only loads when inside canvas. This makes it impossible for other plugins (like visualize) to use any of the functions registered by canvas.

preferably we would have a special uiExport for registering this functions, but as a workaround we could use a hack uiExport to load them.

cc @chrisdavies

[elasticmachine]

Pinging @elastic/kibana-app

[elasticmachine]

Pinging @elastic/kibana-canvas