feat(core): adding register/unregisterForgeConfigForDirectory to utils

[IIIMADDINIII]

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Summary:

Fixes: #3510

Currently it is not possible to provide a forge config through the API of @electron-forge/core.
This makes integrating electron forge in to other build systems difficult, if these build systems want to provide the configuration for electron forge (for example by using metadata from previous build steps).

This pull request implements two functions utils.registerForgeConfigForDirectory(dir: string, config: ForgeConfig): void and unregisterForgeConfigForDirectory(dir: string): void to make this easier.
A "virtual" config registered through the first method behaves as if a forge.config.js file where present at that location exporting the object provided by the config parameter.
This takes precedence over any existing config options in an existing forge-config.js or package.json file in that directory.

This is my first contribution to this Project and i do not have a lot of experience in Open Source Development.
So If i forgot anything or i should have done something differently, please let me know.
Thank You!!