Add option to control behavior when rejecting new file diffs

This PR adds a new configuration option on_new_file_reject that controls what happens when a user rejects a diff for a new file:

• keep_empty (default): Leaves the empty buffer in place
• close_window: Closes the placeholder split window

Previously, when rejecting a new file diff, the plugin would always try to delete the original buffer. This could cause issues when the buffer was reused from an existing empty buffer, leading to potential crashes when attempting to reopen a diff.

The PR includes:

• New configuration option in config.lua with validation
• Type definition in types.lua
• Logic in diff.lua to track whether the original buffer was created by the plugin
• Only delete the original buffer on reject if it was created by the plugin
• Unit test to verify the fix works correctly

This change improves stability when working with new files and gives users more control over the UI behavior.