fix double esbuild-register register

[ndelangen]

Sometimes the esbuild-register has already happened.

This can be caused by some script having registered it, then calling serverRequire, but it also can happen in unit-tests where modules get re-initialized.

• I added detection for when there's already a handler for .ts files. If that's the case, skip registering esbuild-register.

This seems like a good enough heuristic, but I'm open to changes.
I was inspired to this solution by looking at the esbuild-register source-code.