fix(font): don't break directives with inserted next/font import (#72350)
the `next_font_loaders` transform replaces imports from `next/font` with
slightly different ones. unfortunately it was inserting these imports
**at the top** of the file, which broke the `server_actions` transform
because it expects "use server"/"use cache" directives to be the first
item in the file.
this PR changes the logic in the font transform to insert the import in
the place where the original `next/font` import occurred instead of at
the top of the file.
(i suppose that we could just reorder the transforms and run
`server_actions` before `next_font_loaders`, but this feels more robust)
Fixes #72179