Add support for 11ty.js Virtual Templates

[zachleat]

Related: #1612

[zachleat]

Shipping with 3.0.0-alpha.15. Pass in the raw JavaScript as the virtual template content, like so:

eleventyConfig.addTemplate("virtual.11ty.js", function(data) {
	return `this is a test ${data.page.url}.`;
});

eleventyConfig.addTemplate("virtual.11ty.js", {
  data: () => ({ var: 2 }),
  render: function(data) {
    return `this is a test ${data.var}.`;
  }
});

[VividVisions]

Awesome! So, I guess using JS classes as templates (with data and render methods) will also be as easy as this:

import MyTemplateClass from 'mytemplate.js';

const template = new MyTemplateClass({ options: { /* ... */}});
eleventyConfig.addTemplate('virtual.11ty.js', template);

[zachleat]

@VividVisions any of the 11ty.js types would work there: https://www.11ty.dev/docs/languages/javascript/

[zachleat]

Temporary docs preview URL building here: https://11ty-website-git-v3-11ty.vercel.app/docs/virtual-templates/#javascript