Get FQBN from sketch / workspace configuration

[spoenemann]

Currently the board is hard-coded:
https://github.com/bcmi-labs/arduino-language-server/blob/c32ed1943cba708fc05f3724bb3cf23c8e95390f/handler/handler.go#L161-L163

This identifier should be retrieved from the board selected in the IDE.

[spoenemann]

I'll evaluate using an additional JSON-RPC message for this.