Add `dist-custom-elements` output target to Components package

[alex-ketch]

This will allow using component classes from other projects, which is currently a requirement for implementing a custom ProseMirror NodeView in the stencila/web client package.

[alex-ketch]

Closing this for now as I was able to achieve the same goal in a different way.
By registering the @stencila/components package in the global environment, type definitions were then correctly resolved when creating Element fragments.

E.g.

import '@stencila/components'

const editorEl = document.createElement('stencila-editor')
// editorEl is resolved as HTMLStencilaEditorElement