[fetch-esm] Address CI flakes and Node warning messages

[mjameswh]

What was changed

On the fetch-esm sample:

• Introduce a local, non-fetch workflow variant.
• Use tsx instead of ts-node.
• Use .ts file extensions in import directives instead of .js.

Why

• The fetch-esm sample makes HTTP requests to httpbin.org, which is sometimes slow or flaky. This is rippling down on the temporalio/sdk-typescript repo, as the fetch-esm sample is run as part of temporalio/sdk-typescript's CI tests to validate that the SDK can be used with ESM projects.
• The ts-node library has not seen a commit in more than two years and is no longer aligned with latest releases of Node. This has been causing warning messages when running the fetch-esm sample. Switching to tsx resolves these warnings.