Sapper export: when copying static files, also set a(ccess)time and m(odification)time for the files

[ribizli]

Is your feature request related to a problem? Please describe.
During export static files are copied using Node.js fs module (into __sapper__ folder). To optimize sync to a deployment copying atime and mtime should be done.

Describe the solution you'd like
Use fs.utimesSync to preserve static file's access and modification times. Extending the copy function in utils is good to go.

Describe alternatives you've considered
None

How important is this feature to you?
Quite important, my FTP sync tool supports using only mtime (no hashing, etc).

Additional context
None

[Conduitry]

Implemented in 0.27.13.