Support for setting minimum log level

[tintoy]

Currently, the built-in logger (LanguageServerLogger) for LanguageServer has no way to set a minimum logging level, and so all (even diagnostic-level) messages are unconditionally forwarded to the LSP client via window/logMessage.

Ideally, there would be a way to control the log level. Perhaps via a mutable property on LanguageServer?

CC: @david-driscoll

[david-driscoll]

Makes sense to me.

[tintoy]

Ok will work up a PR :)

[tintoy]

Thanks!