fix(css): preserve dollar signs in external @import urls with lightningcss

[shulaoda]

With the lightningcss transformer, an external @import URL containing $ could be corrupted in the output.

Problem

• After lightningcss analyzes dependencies, placeholders are swapped back to real URLs. The import case used css.replace(dep.placeholder, dep.url) with dep.url as a replacement string.
• In a replacement string, $&, $`, $', $n, $$ are special patterns. So a URL like https://cdn.example.com/a.css?q=$& gets mangled — $& is expanded to the matched placeholder text instead of being inserted verbatim.
• The adjacent url case already guards against this with a function replacer; the import case did not.

Fix

• Use a function replacer (() => dep.url) so the URL is inserted literally, with no replacement-pattern interpretation.

Tests

• Added a preprocessCSS test using the lightningcss transformer with an external @import URL containing $&. It fails without this change (the URL is mangled) and passes with it.