Support template as promise

[thantos]

Other single-spa plugins support promises. Making this one consistent. Lifecycle methods already nicely support promises.

singleSpaHtml({
    template: () => fetch(...).then(x => x.text()),
})

Tested this on single-spa core using...

export const mount = [async (props) => {
     const elm = props.domElementGetter && props.domElementGetter(props);

     if(elm) {
         elm.innerHTML = await fetch(...).then(x => x.text());
    }
}]  

[joeldenning]

Thanks, this is a sensible change. Could you also document it, too? Here's the file: https://github.com/single-spa/single-spa.js.org/blob/master/website/versioned_docs/version-5.x/ecosystem-html-web-components.md

[joeldenning]

Released in https://github.com/single-spa/single-spa-html/releases/tag/v1.3.0. I also invited you to have write access to the single-spa-html repo, since you've helped out a couple times with it now. If you have interest in helping maintain it, you can watch the repo and respond to issues/pull requests as they come in. This repo doesn't see a lot of issues or proposed changes, but I'd appreciate your help with it.