[plugin] Enhance features of LogOutputChannelImpl

[ndoschek]

Feature Description:

Follow-up of #12017 (PR #12429):

The initial PR contributed the basic VS Code API support for LogOutputChannels, to support extensions to make use of the LogOutputChannel. VS Code offers some additional features which would also be great for Theia:

• Currently, the log level for LogOutputChannels is set by default to INFO and cannot be changed. VS Code supports to set the log level via the command Developer: Set Log Level... in the application which also configures the log level for LogOutputChannels.
• Also, VS Code writes the content of the LogOutputChannel to a separate log file per extension (see also command Developer: Open Extension Logs Folder) and offers a tool in the output view toolbar to directly open the extensions's log file in the application.