Constructors rather than factories

[Rich-Harris]

Having gone back and forth on this, I think it makes more sense for components to be constructors rather than factories – i.e. instead of this...

export default function createComponent ( options ) {
  // ...
}

...this:

export default function Widget ( options ) {
  // ...
}

This makes Svelte components slightly more familiar for people coming from e.g. Ractive or Vue, and has this important benefit:

// awkward naming
import createWidget from './Widget.html';
const widget = createWidget(...);

// better
import Widget from './Widget.html';
const widget = new Widget(...);

(Capitalising filenames makes sense because inline components should also be capitalised, i.e. <Widget foo='bar'/> – this is a good way to visually distinguish elements from components in a way that doesn't make them look like web components.)