doc(test-runner): add example of compiling tests with typescript (#8046)

docs/src/test-typescript-js.md

@@ -0,0 +1,34 @@
+## Manually compile tests with TypeScript
+
+Playwright Test supports TypeScript out the box. We automatically transform
+TypeScript code to javascript to run it.
+
+However if you find that the TypeScript code is not being transpiled correctly,
+you can perform your own TypeScript compilation before sending the tests to Playwright.
+
+First I add a `tsconfig.json` file inside my tests directory.
+```json
+{
+    "compilerOptions": {
+        "target": "ESNext",
+        "module": "commonjs",
+        "moduleResolution": "Node",
+        "sourceMap": true,
+        "outDir": "../tests-out",
+    }
+}
+```
+
+In my `package.json`, I have two scripts:
+```json
+{
+  "scripts": {
+    "pretest": "tsc --incremental -p tests/tsconfig.json",
+    "test": "playwright test -c tests-out"
+  }
+}
+```
+
+The `pretest` script runs typescript on the tests. `test` will run the tests that have been generated to the `tests-out` directory. The `-c` argument configures the test runner to look for tests inside the `tests-out` directory.
+
+Then `npm run test` will build the tests and run them.