feat(fs): add copy/copySync

[axetroy]

part of #261

[axetroy]

I am implementing copy link file

Here's a problem:

1. Deno does not have API to update files creation time now. Feature Request: Deno.utimes() api deno#1948
2. Deno does not have an API to delete the link file Deno.remove() can not remove symlink file deno#1947

[axetroy]

/cc @ry ready for review

[axetroy]

By the way

Because the test files are too large and difficult to review.

Is it possible to separate synchronous and asynchronous tests?

// copy_test.ts
import "./copy_async_test.ts"
import "./copy_sync_test.ts"

[ry]

@axetroy I don't mind big files. I'd prefer you keep related code together. But I leave it up to you.

[ry]

LGTM - nice work!