`files` manifest option should warn when `destination` does not appear to be a directory

Given the following manifest configuration:

files = [{ source = "static/foo.txt", destination = "bar.txt" }]

I would expect at least a warning as I believe destination must always be a directory. If source is a file, that file will be located inside the destination directory, and if source is a directory, all the files will be available at destination. I imagine we can't always error because paths that look like file names are still legal directory names (i.e., "foo.txt" can be a directory name), but we can at least issue a warning as this is more than likely a bug.

Edit: it seems that destinations as files are allowed, but this will break in very weird ways if source is a directory and not a file.

Start in crates/loader/src/local.rs around the destination handling referenced in the issue, then reproduce the manifest with a file source and a directory source. Confirm how a destination that looks like a filename is handled and add coverage for the requested warning; done means the directory case warns without rejecting legal directory names.