Placeholder should not operate on the actual text buffer

[databasedav]

change detection on text triggers when the placeholder is set, which doesn't make sense, since the text that we care about is actually empty

instead, the placeholder should just insert a vanilla bevy text node when the buffer is empty, or CosmicTextChanged should have an attribute for placeholder text, although the former is highly preferred albeit more difficult to implement due to needing to synchronize the cosmic text and bevy styling