fix: always copy manifest file

[ksoda]

Current behaviour:

1. run webpack as watch mode
2. modify some file
3. reload chrome extension
4. get an error caused by lack of manifest in the build directory

The copyUnmodified option fixes this problem

Out of scope: JS Style format

[ksoda]

This is already described in #71 (comment) ;)

[ksoda]

I think the #73's approach is better than this one. That change may achieve a higher performance. 🤔

[rayronvictor]

I think the #73's approach is better than this one. That change may achieve a higher performance. 🤔

#73 approach does not solve the problem of having to copy an entire folder, as when using _locales, while { copyUnmodified: true } does.