LSP server ignores backend setting in configuration

As a user of the VSCode extension, I want a change to the backend setting to take immediate effect so that I don't have to restart the extension.

Our VSCode plugin already sends the `backend` string as part of the configuration (e.g. via LSP `DidChangeConfigurationParams`). However, currently, `compileSource` takes an `EffektConfig` which is an immutable `ScallopConf`. It is not intended to change the `EffektConfig` once it has been parsed.

As a solution, we want to separate the CLI arguments from the configuration tracked internally. For this I propose the following steps:

* [ ] Disentangle configuration from kiama
* [ ] Separate CLI arguments and internal configuration
* [ ] Change the `backend` field on the internal (mutable) configuration