fix: Wrap vite-plugin-external in custom plugin and fix issues

[stevezhu]

Issue: #20643

Alternate solution for: #20698

What I did

HMR was broken by vite-plugin-externals due to returning the entire config from the config hook instead of the partial config as written in the vite api docs: https://vitejs.dev/guide/api-plugin.html#vite-specific-hooks.

Recreates the same fix as in crcong/vite-plugin-externals#27 by wrapping the plugin in a custom plugin and overriding the result of the config hook.

How to test

1. Run a sandbox for template, e.g. yarn task --task sandbox --start-from auto --template react-vite/default-ts
2. Build @storybook/builder-vite, yarn build --watch builder-vite from code/ directory
3. Open Storybook in your browser
4. Ensure HMR is working
5. Ensure there is not a regression with [Bug]: Investigate code getting pre-bundled multiple times #20514

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"]

[stevezhu]

Performance test using https://github.com/Shinigami92/vite-plugin-time-reporter

Ran a few times with the sandbox build.

Results seem comparable. I would imagine most of the build time is going towards other parts of of the build process.

This PR (#20770)

(index)	time
build	11.124s
render	3.146s
full	20.196s

(index)	time
build	14.477s
render	4.086s
full	24.790s

(index)	time
build	13.324s
render	3.856s
full	23.133s

#20698

(index)	time
build	14.411s
render	5.921s
full	26.623s

(index)	time
build	10.516s
render	3.139s
full	19.009s

(index)	time
build	11.514s
render	3.339s
full	19.431s

[stevezhu]

Opted for #20698

[Dschungelabenteuer]

@stevezhu thanks again for all your constructive feedback, appreciated! 🤗