Preserve execute permissions

Feature request. Could the 'execute' permission be preserved the the generated output?

Current situation:
```sh
ls -l src
-rwxr-xr-x 1 user staff 656 Jun  4 19:42 main.ts

ls -l build
-rw-r--r-- 1 user staff 788 Jun  4 19:56 main.js
```
It would be nice if `build/main.js` also had the `x` permissions so it can be executed directly.
