Storysource: Fallback to the docs.source.originalSource parameter

[shilman]

Closes #20421

What I did

Fallback to us the source provided by the docs.source.originalSource parameter.

There is a long history to this hack of a PR. The TLDR is that:

1. This data should be provided by source-loader in the storySource.source parameter. However, source-loader caused a performance regression in the Vite builder, so I reverted it.
2. In Vite, csf-plugin runs always (currently), so we can expect the docs.source.originalSource to be there.

The addon will prefer storySource.source for legacy support, but also use docs.source.originalSource if it's not there.

Note that in webpack:

• source-loader IS run
• csf-plugin IS NOT run unless addon-docs is enabled

Hopefully this all gets replaced with the annotation server in 7.x...

How to test

Run a Vite sandbox with the storysource addon:

yarn task --task dev --template react-vite/default-ts -a storysource

Verify that the code shows up in the Code addon panel.