fix: Mirror the versions used by pug-plain-loader

[df-pkennedy]

prevents npm from choking when Pug 3 is installed, which is happily handled despite NPM suggesting otherwise

While resolving: vuetify-loader@1.9.0
Found: pug@3.0.2
node_modules/pug
  dev pug@"^3.0.2" from the root project
  peer pug@"^2.0.0 || ^3.0.0" from pug-plain-loader@1.1.0
  node_modules/pug-plain-loader
    dev pug-plain-loader@"^1.1.0" from the root project

Conflicting peer dependency: pug@2.0.4
node_modules/pug
  peerOptional pug@"^2.0.0" from vuetify-loader@1.9.0
  node_modules/vuetify-loader
    dev vuetify-loader@"^1.9.0" from the root project