types: add apply property

[garand]

TypeScript is complaining when using this plugin with this error:

Property 'apply' is missing in type 'WebpackPwaManifest' but required in type 'WebpackPluginInstance'

html-webpack-plugin had this same problem and was solved with this PR.

I've mirrored that change here, and manually in my node_modules/webpack-pwa-manifest/index.d.ts file for testing, and it has fixed the error.