FileWatching: prevent double-close of FileMonitor/FolderMonitor

The `close` function for FileMonitor/FolderMonitor reads the handle and
calls `jl_close_uv`, but does not mark the handle as closed. If `close`
is called twice before the async close callback (`_uv_hook_close`) has
a chance to set handle to C_NULL, `jl_close_uv` would be called twice
on the same handle, hitting a `uv_is_closing` assertion in libuv.

Fix by checking `uv_is_closing` before calling `jl_close_uv` to skip
handles that are already in the process of being closed.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>