Refactor launch_url and copy_to_clipboard

See original discussion here:
https://github.com/flutter/devtools/pull/6598#discussion_r1373364646

both essentially use postMessage, so if we created a config_specific implementation of postmessage, then they could use that instead of both creating a bunch of stub files.

Start by reading the linked discussion and locating launch_url, copy_to_clipboard, and their postMessage usage. The refactor is done when both use a config-specific postMessage implementation instead of creating separate stub files.

Written by the indexing model from the issue text.