Add expectElement()

[fregante]

Closes #20

[fregante]

Will be used in https://github.com/refined-github/refined-github/pull/6973/files#diff-c95e377cf1c957e2609d26792bac720c60f2cff7e2b2fadec703a5dc5764ab2fR13-R22

Unfortunately thrown errors can't specify the original feature like features.log.error can, unless thrown in the init

I think the only way to achieve this would be to pre-wrap every callback to provide the source, .e.g.

observe(selector, wrap(import.meta.url, addButton));

function addButton(parent) {
	parent.append(<button/>)
}

// utils.js
function wrap(feature, callback) {
	return (...args) =>  {
		try {
			callback(...args)
		} catch (error) {
			features.log.error(feature, error);
		}
	}
}

I think this would work, if we remember to wrap each and every callback 😬

This is where decorators would be useful, too bad they're only available to classes 🤦‍♂️

@callback(import.meta.url)
function addButton(parent) {
	parent.append(<button/>)
}

cc @yakov116 @134130

[134130]

LGTM

[yakov116]

Looks great