Extend tsconfig.json from SvelteKit.

tsconfig.json

 {
-  "compilerOptions": {
-    "moduleResolution": "node",
-    "module": "es2020",
-    "lib": ["es2020", "DOM"],
-    "target": "es2019",
-    /**
-      svelte-preprocess cannot figure out whether you have a value or a type, so tell TypeScript
-      to enforce using \`import type\` instead of \`import\` for Types.
-      */
-    "importsNotUsedAsValues": "error",
-    "isolatedModules": true,
-    "resolveJsonModule": true,
-    /**
-      To have warnings/errors of the Svelte compiler at the correct position,
-      enable source maps by default.
-      */
-    "sourceMap": true,
-    "esModuleInterop": true,
-    "skipLibCheck": true,
-    "forceConsistentCasingInFileNames": true,
-    "baseUrl": ".",
-    "allowJs": true,
-    "checkJs": true,
-    "paths": {
-      "$lib": ["src/lib"],
-      "$lib/*": ["src/lib/*"]
-    }
-  },
-  "include": ["src/**/*.d.ts", "src/**/*.js", "src/**/*.ts", "src/**/*.svelte"]
+  "extends": "./.svelte-kit/tsconfig.json"
 }