Webpack 5 support

[Eli-Black-Work]

Preflight Checklist

• I have read the contribution documentation for this project.
• I agree to follow the code of conduct that this project follows, as appropriate.
• I have searched the issue tracker for a feature request that matches the one I want to file, without success.

Problem Description

Electron Forge supports Webpack 4 but not Webpack 5.

Our project is currently using Electron Forge with Webpack 5, so we have some experience with this.

Things that currently work:

• electron-forge start
• electron-forge package
• Some plugins:
  • style-loader
  • css-loader
  • babel-loader
  • svg-sprite-loader
  • svgo-loader

Things that currently don't work:

• At least some of the plugins that ship with Webpack 5
  • Such as Webpack 5's DefinePlugin ([v6] doesn't work with Webpack 5's DefinePlugin #2177)
• Possibly file-loader.
  • We have CSS that loads custom fonts, which are loaded via file-loader, and these aren't working.

There is currently a PR open for this, but I'm not sure how complete it is: #2225