Vite: Fix storysource addon support

[shilman]

Closes #20421

Self-merging @tmeasday @joshwooding

What I did

This PR:

• restores/fixes storysource support for the vite builder.
• fixes extra addon handling in our development sandboxes

There's a bit of history behind this PR. Once upon a time, addon-docs and addon-storysource both depended on a library called source-loader to load & display raw story source.

1. I moved addon-docs to a new library called csf-plugin here: Addon-docs: Replace source-loader with csf-plugin #19680
2. @joshwooding added csf-plugin support for Vite and removed source-loader here: Vite: Fix static source handling for addon-docs #20147
3. I changed the output of csf-plugin and updated addon-docs in a way that broke addon-storysource Csf-plugin: Move source to docs.source.originalSource #20665
4. I made various fixes to source-loader/storysource to get things working in webpack, e.g. Source-loader: Fix export default variable references #20688
5. With this change, addon-storysource and addon-docs are completely separated in both vite & webpack.

The plan is to:

• Potentially move source handling to an "annotations server" that's part of our 7.x/8.0 roadmap
• Create a better replacement for storysource that shares code with addon-docs
• Deprecate storysource/source-loader, and sunset them in 8.0

How to test

1. Run a sandbox for template, e.g. yarn task --task sandbox --start-from auto --template react-vite/default-ts --addon storysource
2. Open Storybook in your browser
3. Access the Button stories and view the Story tab in the addons panel

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"]

[joshwooding]

Nice work!