Support multiple output channels for log message notifications

Proposing a feature to allow log message notifications to report their output to a specific channel. These could be used to differentiate the output between multiple long and short lived tasks that may be run by the same language server at any given time.

Currently, since all log message notifications end up mixed in the same output channel, it can be difficult to use for cases such as implementing a run of a build that produces its own linear output that should be collected and viewed separately from any of the server's other output.

Another option could be something more like a show document request that displays text in a more read only manner, for example in a display more similar to the view in which VS code outputs test run progress.

Start by reading the protocol's log message notification behavior and the alternative show document request described in the issue, then compare how VS Code presents test-run progress. Define the protocol scope and expected channel behavior before identifying the affected specification entry points and validation needed.