Add Metalsmith.imports method

[webketje]

Proposed signatures:

async function imports(module:string):Promise<ModuleObject>
function imports(module:string, callback: (err: Error|null, ModuleObject) => void

The method allows loading of JS, CJS, MJS modules

A variant of this is already in use at:

• metalsmith CLI code: https://github.com/metalsmith/metalsmith/blob/v2.6.2/bin/metalsmith#L79
• @metalsmith/in-place https://github.com/metalsmith/in-place/blob/v5.0.0/src/utils.js#L41
• (implicitly) @metalsmith/layouts https://github.com/metalsmith/layouts/blob/v2.7.0/src/get-transformer.js

And will potentially be needed for @metalsmith/markdown...
so it is only natural to provide alignment through a helper method