Allow to programmatically define parcel config for the API

[CyriacBr]

🙋 feature request

I've been testing and benchmarking some new bundlers to replace a very old frontend project and Parcel is the only one that doesn't let you define all configurations programmatically. For example, no way to pass transformers to Parcel constructor...
That feature seems to have been removed because of #4927.

🤔 Expected Behavior

Parcel should let you pass all configs as an object like all other bundlers do.

😯 Current Behavior

config expects a path to read a static file.

💁 Possible Solution

One very ugly solution is to create a temp file with the config you want before calling Parcel.

🔦 Context

Building code programmaticaly.

💻 Examples

new Parcel({
  config: {
    // whatever config you want
  },
})