Relocate default template engine from x-element.js to x-template.js

[klebba]

In order to better organize the two discrete classes we will relocate the template engine to a separate file (rather than just a separate class)

Consider a means of preventing the default template engine from being loaded at all.

Perhaps detect special options as query params on the import itself, e.g.:

import XElement from 'path/to/x-element.js?dont-do-it';

Importantly we do not want to require a change in the manner in which the library is included — it should always only require import XElement from 'path/to/x-element.js';