Custom webpack

[idastambuk]

What this PR does / why we need it:
Hey all, I ran create-plugin update in one of our plugins that had a webpack.config.ts in its root with some plugin-specific loaders. The script overwrote them to use the ./config/webpack.config.ts, which made some build steps in the CI fail.
I'm not sure if this is the best way to do it, but I tried to add a check that, if the plugin already has a webpack.config.ts file, it will use that file in build and dev scripts.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[github-actions]

Hello! 👋 This repository uses Auto for releasing packages using PR labels.

✨ This PR can be merged but will not trigger a new release. To trigger a new release add the release label before merging.
NOTE: When merging a PR with the release label please avoid merging another PR. For further information see here.

[jackw]

@idastambuk we really appreciate you looking into a fix for this. It's a known issue that was reported here. However due to how the update command currently operates and, as you've witnessed, breaks things in multiple ways we've decided to rewrite it so it's smarter when it comes to making changes.

Given we already have a way forward that be implemented soon and will solve this issue (along with the others listed in the epic) I'm going to go ahead and close this.