Adopt to new vscode log output channel API

[sandy081]

https://github.com/microsoft/vscode/blob/ed1bc56bd8b6343296ba8310167928e1e955a74e/src/vscode-dts/vscode.proposed.extensionLog.d.ts#L1

[dbaeumer]

See https://github.com/microsoft/vscode-languageserver-node/tree/dbaeumer/adaptLogChannel

[dbaeumer]

I recalled all the discussion we had and I looked into making this backwards compatible with the settings I introduced. However this is quite some work for no real benefit. So I decided to make this a breaking change to align with VS Code's behavior and ship it in 3.18.

[JustinGrote]

@dbaeumer following up on this, are there any major issues pending or anything I can contribute (tests, etc.) to move this along? For the PowerShell extension we really would like our LSP server messages to have severity filtering available.

[JustinGrote]

Alternatively, have you considered implementing something like the DebugAdapterTrackerFactory in vscode?
https://vscode-api.js.org/interfaces/vscode.DebugAdapterTrackerFactory.html

As an author, I find this the most useful as I can control how messages from the client are logged.