adjust HAS_IMPORT regex (#1062)

fix #1060 

match RegExp `HAS_IMPORT` with `fileMayBeTailwindRelated()`

---

```js
let HAS_IMPORT = /@import\s*('[^']+'|"[^"]+");/
let HAS_IMPORT = /@import\s*['"]/ // from fileMayBeTailwindRelated()
```

It seemed like the regex only accepted `@import '~~~';`, so `@import
'~~~' layer(xxx);` did not match.
Therefore, I loosened the regex of `HAS_IMPORT` a bit so that it would
match the other parts.

<sub>translated from japanese</sub>

Author: Kim Hokyeong

packages/tailwindcss-language-server/src/project-locator.test.ts

@@ -152,6 +152,19 @@ testFixture('v4/auto-content', [
   },
 ])
 
+testFixture('v4/auto-content-split', [
+  //
+  {
+    config: 'src/app.css',
+    content: [
+      '{URL}/package.json',
+      '{URL}/src/index.html',
+      '{URL}/src/components/example.html',
+      '{URL}/src/**/*.{py,tpl,js,vue,php,mjs,cts,jsx,tsx,rhtml,slim,handlebars,twig,rs,njk,svelte,liquid,pug,md,ts,heex,mts,astro,nunjucks,rb,eex,haml,cjs,html,hbs,jade,aspx,razor,erb,mustache,mdx}',
+    ],
+  },
+])
+
 testFixture('v4/custom-source', [
   //
   {

packages/tailwindcss-language-server/src/project-locator.ts

@@ -661,7 +661,7 @@ class FileEntry {
   isMaybeTailwindRelated(): boolean {
     if (!this.content) return false
 
-    let HAS_IMPORT = /@import\s*('[^']+'|"[^"]+");/
+    let HAS_IMPORT = /@import\s*['"]/
     let HAS_TAILWIND = /@tailwind\s*[^;]+;/
     let HAS_DIRECTIVE = /@(theme|plugin|config|utility|variant|apply)\s*[^;{]+[;{]/
 

packages/tailwindcss-language-server/tests/fixtures/v4/auto-content-split/package-lock.json

@@ -0,0 +1,6 @@
+{
+  "dependencies": {
+    "tailwindcss": "^4.0.0-alpha.25",
+    "@tailwindcss/oxide": "^4.0.0-alpha.25"
+  }
+}

packages/tailwindcss-language-server/tests/fixtures/v4/auto-content-split/package.json

@@ -0,0 +1,3 @@
+@import "tailwindcss/preflight" layer(base);
+@import "tailwindcss/theme" layer(theme);
+@import "tailwindcss/utilities" layer(utilities);

packages/tailwindcss-language-server/tests/fixtures/v4/auto-content-split/src/app.css

@@ -0,0 +1 @@
+<div class="underline">Test</div>

packages/tailwindcss-language-server/tests/fixtures/v4/auto-content-split/src/components/example.html

@@ -0,0 +1 @@
+<div class="flex">Test</div>