feat: add "resolver" options to be able to use a different cdn

[mastilver]

options.resolver

Type: string || function

Default: 'module-to-cdn' || 'module-to-jsdelivr'

Allow you to define a custom module resolver, it can either be a function or an npm module
The resolver should return either null or an object with the keys: name, var, url

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Related to #3, but following a different / more modular logic