Enable viteExternalsPlugin in dev mode

[valentinpalkovic]

Issue: N/A

Enable viteExternalsPlugin for dev mode.

What I did

How to test

Checklist

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests)
• Make sure to add/update documentation regarding your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Maintainers

• If this PR should be tested against many or all sandboxes,
  make sure to add the ci:merged or ci:daily GH label to it.
• Make sure this PR contains one of the labels below.

["cleanup", "BREAKING CHANGE", "feature request", "bug", "documentation", "maintenance", "dependencies", "other"]

[valentinpalkovic]

Hi @crcong,

we have some issues with your plugin related to HMR and we don't know how to solve it. If we remove the disableInServe then HMR is broken but we do not know why. Can you help us to investigate that further?