`file` loader should copy original timestamps

I am using `--loader=file` for some outputs which are further post-processed by `make` build tool.
The problem is esbuild always rewrites output files, even when inputs are unchanged. This leads to new timestamps and unnecessary rerunning of costly post-processing.

Ideally, no rewriting should occur for input files which are unchanged.
But copying file attributes like modification time will also work.

Start at the `--loader=file` path and reproduce the behavior with outputs consumed by `make`. Compare timestamps for outputs whose inputs are unchanged; done means those inputs no longer trigger unnecessary post-processing, either by avoiding rewrites or preserving file attributes.

Written by the indexing model from the issue text.