Reify language backends, introducing LanguageBackendRegistry

* Many case differentiations in the agent code were replaced by
  method calls in the newly introduced LanguageBackend abstraction
* The registry allows new backends to be added dynamically
  (via Python packages that implement a specific entrypoint)

scripts/demo_diagnostics.py

@@ -14,8 +14,9 @@ from pathlib import Path
 from pprint import pprint
 
 from serena.agent import SerenaAgent
-from serena.config.serena_config import LanguageBackend, ProjectConfig, RegisteredProject, SerenaConfig
+from serena.config.serena_config import ProjectConfig, RegisteredProject, SerenaConfig
 from serena.constants import REPO_ROOT
+from serena.language_backend import BuiltinLanguageBackend
 from serena.project import Project
 from serena.tools import (
     CreateTextFileTool,
@@ -35,7 +36,7 @@ def make_agent() -> SerenaAgent:
     """Create an LSP-backed Serena agent for the Serena repository."""
     serena_config = SerenaConfig.from_config_file()
     serena_config.web_dashboard = False
-    serena_config.language_backend = LanguageBackend.LSP
+    serena_config.set_builtin_language_backend(BuiltinLanguageBackend.LSP)
 
     project = Project(
         project_root=str(REPO_PATH),