test(svelte-3): add dedicated tests

Add test scaffolding for Svelte 3.59 to ensure maximum compatibility
across all supported Svelte versions (3, 4, 5 non-Runes mode).

Author: metonym

.github/workflows/checks.yml

@@ -36,6 +36,20 @@ jobs:
       - name: Test Svelte 4
         run: bun run test
 
+  test-svelte3:
+    runs-on: macos-15
+    steps:
+      - uses: actions/checkout@v6
+      - uses: oven-sh/setup-bun@v2
+      - run: bun ci
+
+      - name: Test Svelte 3
+        run: |
+          cd tests-svelte3
+          bun ci
+          bun run test
+          bun run test:types
+
   test-svelte5:
     runs-on: macos-15
     steps:

package.json

@@ -33,6 +33,8 @@
     "test": "vitest",
     "test:src-types": "tsgo",
     "test:types": "svelte-check --workspace tests --fail-on-warnings",
+    "test:svelte3": "cd tests-svelte3 && bunx vitest",
+    "test:types:svelte3": "cd tests-svelte3 && bun run test:types",
     "test:svelte5": "cd tests-svelte5 && bunx vitest",
     "test:types:svelte5": "cd tests-svelte5 && bun run test:types",
     "lint": "biome check --write .",