change webpack devtool to cheap-source-map

[zicsus]

What does this PR do?
Changes webpack devtool to more performant cheap-source-map. This will help speed up compilation a bit although we should consider giving user an option to add custom webpack config. Lower config systems should benefit from this.

Where to start reviewing?
gulpfile.js

Related Issues?
n/a

Caveats?
n/a

Reviewers?
@nkrusch

[nkrusch]

@zicsus unrelated to this PR but about source maps in general:

I noticed there is no way to disable source maps for prod builds. I think this should be an option, because it reduces extension size and obfuscation reasons. What are your thoughts?

[zicsus]

@nkrusch is it generating source maps in production build?
We are setting mode properly so not sure why it's doing that. I've to take a look.

[nkrusch]

Yes. I tried latest and double checked; I'm seeing this behavior.

[zicsus]

Yeah! It is doing that. I'll fix it.