[types] Use of webpack-5-only type breaks builds

[lobsterkatie]

Two years ago, we modified the errorHandler option to take a third argument, of the type Compilation. Though the contents of the type exists in similar forms in webpack 4 and 5, in webpack 4 it's exported as compilation.Compilation, where as in webpack 5, it's exported at the top-level.

This didn't matter much until about a month ago, when #308 updated the overall options type to reflect the addition of the third argument for errorHandler. That change was released two days ago, in v 1.17.2. Because that PR used Compilation in its webpack 5/top-level form, anyone using this plugin together with webpack 4 and TS will receive the following error when they import the plugin (assuming they have skipLibCheck set to false in their TS config):

Steps to Reproduce

1. Create a TS project with webpack@4.x, @types/webpack@4.x, and @sentry/webpack-plugin@1.17.2 as dependencies
2. Import the plugin (import { default as SentryWebpackPlugin } from "@sentry/webpack-plugin";)
3. Set skipLibCheck to false in the TS config
4. Run tsc

Expected Result

No error

Actual Result

The error above