Lazy load the js

## 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

Compare the lazy-loading approach in ipycytoscape pull request 222, then inspect ipympl's js/src/index.js and its package.json version handling. Determine how widgetExports can be loaded lazily without breaking version access; done means the JupyterLab splash screen can appear sooner while the widget still initializes correctly.

Written by the indexing model from the issue text.