Webpack plugin v2

[lforst]

### Steps
- [ ] https://github.com/getsentry/sentry-javascript-bundler-plugins/pull/216
- [ ] https://github.com/getsentry/sentry-javascript-bundler-plugins/pull/218
- [ ] https://github.com/getsentry/sentry-javascript-bundler-plugins/pull/230
- [ ] https://github.com/getsentry/sentry-javascript-bundler-plugins/pull/232
- [ ] https://github.com/getsentry/sentry-javascript-bundler-plugins/pull/231
- [ ] https://github.com/getsentry/sentry-javascript-bundler-plugins/pull/237
- [x] Cut alpha
- [x] Write down migration path
- [x] Ship
- [x] Add note to old repo that it now lives here