CSF-plugin: Fix sourcemaps

[shilman]

Closes N/A

What I did

Added sourcemap support for CSF-plugin using Babel's built in sourcemap support.

How to test

In a Vite project before the fix, it will output the following warnings:

the transformation. Consult the plugin documentation for help
rendering chunks (108)...Sourcemap is likely to be incorrect: a plugin (unplugin-csf) was used to transform files, but didn't generate a sourcemap for the transformation. Consult the plugin documentation for help
rendering chunks (114)...Sourcemap is likely to be incorrect: a plugin (unplugin-csf) was used to transform files, but didn't generate a sourcemap for the transformation. Consult the plugin documentation for help

After the change, the warnings go away.

[IanVS]

It looks like there are some failing CI checks, but otherwise this looks good and indeed solves the missing sourcemap issue in my project.