webpack-plugin: put code to `@griffel/webpack-loader`

[layershifter]

Pre-requisites

• Work has already been done in https://github.com/microsoft/griffel/tree/feat/new-plugin. ALWAYS pick changes from the branch
• We need to slice changes into smaller PRs to review them separately

Task

• Put actual code to @griffel/webpack-plugin from the branch
• Don't add fixtures or any fixture based tests yet