fix: ensure new webpack5 generator property in rules passes ruleset checks (v15)

[LinusBorg]

Webpack 5 has a new property in rule definitions called generator - we need to account for that in the rule validiations we have in place.

Note this is for vue-loader 15, targeting Vue 2. Matching PR for v16: #1754

Tests

I wasn't sure how to test for this as the repo's webpack version is locked to v4, and the property only exists in v5.

I locally changed the webpack version to v5, added the following rule to the test cases base config and ran the tests:

{
  test: /\.tiff$/,
  type: 'asset/resource',
  generator: {
    filename: '[hash][ext][query]'
  }
}

... multiple tests broke without my change, and all tests passed after my change.

close #1729