chore: Remove @types/terser-webpack-plugin dependency

[SimonAlling]

A typical userscript doesn't need it because it doesn't import anything from the terser-webpack-plugin package; I therefore consider this a backward-compatible change.

I did this:

$ npm why @types/terser-webpack-plugin
@types/terser-webpack-plugin@2.2.0
node_modules/@types/terser-webpack-plugin
  @types/terser-webpack-plugin@"^2.2.0" from the root project

$ npm remove @types/terser-webpack-plugin

$ npm install --save-dev --save-exact @types/terser-webpack-plugin@2.2.0