feat(watch mode): toggle webpack development mode on watch

[ajubin]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: 1318

#1318

What is the new behavior?

See issue

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[ajubin]

Hi, I'm not sure how to test my change directly from the forked code.
I've tried my fix on my node_modules in my project though, but since I did it on dist code, it's not exactly the same

Do I need to do something else for documenting ?

[ajubin]

Hi @kamilmysliwiec , is there any reason not to merge this PR ? I can work on it if something is missing :)

[jmcdo29]

LGTM, we'll wait to hear from Kamil, but the change looks straightforward enough

[kamilmysliwiec]

LGTM

This will be added in the next release