Fix typescript declarations to work with webpack@5

[drauggres]

Fix issue:

TS2322: Type 'GeneratePackageJsonPlugin' is not assignable to type '((this: Compiler, compiler: Compiler) => void) | WebpackPluginInstance'.

• webpack@5 has its own ts-declaraions (no need for @types/webpack)
• Plugin is not exported from webpack in webpack@5 (is it WebpackPluginInstance now?)

Though it is possible to remove @types/webpack from the package.json and change parent class to WebpackPluginInstance it will break compatibility with webpack@4.

With this change plugin works with both webpack@4 and webpack@5