Lazy load the js

[ianhi]

Describe the issue

I don't think that the js is loaded lazily which increases the time until the jlab splash screen shows up.

Proposed Solution

Lazy load the widgetExports similar to https://github.com/QuantStack/ipycytoscape/pull/222

Though this may be complicated by the reading of package.json for the version? https://github.com/matplotlib/ipympl/blob/3a75b2bbab58bf5748c87c96c05e79e6545f64e0/js/src/index.js#L4

[jasongrout]

In your lab extension, you could just import that version from package.json directly.

[martinRenou]

👍🏼