Allow overwriting of default web preferences

[JulianGlueck]

This PR will allow the user to overwrite NativePHP's default web preferences passed to Electron.

At the time of creating this PR, these are backgroundThrottling, spellcheck, preload, sandbox, contextIsolation, and nodeIntegration. See details here: https://github.com/NativePHP/laravel/issues/688

[gwleuverink]

Heya! I took a mo to make the preload script work with enhanced security preferences #255

I think we should reassess which of the default preferences may be overwritten without breaking the setup.

Right now in my PR it needs to force the preload, contextIsolation & sandbox preferences. I think the other stuff may be overwritten without issues

[gwleuverink]

I've shuffled some things around and added this in #255. Closing this in favor of that PR

Added you as a co-author here: a1061ff