How would I go about registering widgets?

as the title says, how would I go about registering widgets.

I'm currently using decap although I think it is also possible with sveltia,

Documentation for it in decap is here: <https://decapcms.org/docs/custom-widgets/>

I had a quick look at https://github.com/privatemaker/headless-cms/blob/main/layouts/_default/headless-cms.html and didn't see any way to hook in.

Thanks in advance