feature request: Create common rule for symlinking

It would be great if skylib had a rule similar to `copy_file` that, instead of manually copying the file, uses `ctx.actions.symlink` (since Bazel 3.2).

This could be either implemented as a new rule (`symlink_file`?), or as attribute of `copy_file` (which could potentially become the default behavior in the future).

Start by examining the existing `copy_file` rule and the Bazel 3.2 `ctx.actions.symlink` API. Determine whether the intended result is a separate `symlink_file` rule or an attribute on `copy_file`, then verify that the chosen interface creates symlinks instead of copying files.

Written by the indexing model from the issue text.