Don't attempt to register web component if already created

[timkpaine]

The browser complains loudly and there may be some other side effects (e.g. in perspective's fat webpack bundle when used with other libraries that leverage regular-table, such as ipyregulartable)

[texodus]

Thanks for the PR! Looks good!

I'm compelled to leave for historical record a reminder, that if you encounter issues with regular-table duplicate registering, you have multiple copies of regular-table.js in your bundle and this is never a good sign.