Inject @config "..." when a tailwind.config.{js,ts,...} is detected (#14635)

This PR injects a `@config "…"` in the CSS file if a JS based config has
been found.

We will try to inject the `@config` in a sensible place:
1. Above the very first `@theme`
2. If that doesn't work, below the last `@import`
3. If that doesn't work, at the top of the file (as a last resort)

integrations/utils.ts

@@ -330,7 +330,8 @@ export function test(
                 return a[0].localeCompare(z[0])
               })
               .map(([file, content]) => `--- ${file} ---\n${content || '<EMPTY>'}`)
-              .join('\n\n')}`
+              .join('\n\n')
+              .trim()}\n`
           },
           async expectFileToContain(filePath, contents) {
             return retryAssertion(async () => {