Refactor to unify scripts/categories as Executable

This commit consolidates scripts and categories under a unified
'Executable' concept. This simplifies the architecture and improves code
readability.

- Introduce subfolders within `src/domain` to segregate domain elements.
- Update class and interface names by removing the 'I' prefix in
  alignment with new coding standards.
- Replace 'Node' with 'Executable' to clarify usage; reserve 'Node'
  exclusively for the UI's tree component.

tests/unit/shared/Stubs/ExecutableErrorContextStub.ts

@@ -0,0 +1,11 @@
+import type { ExecutableErrorContext } from '@/application/Parser/Executable/Validation/ExecutableErrorContext';
+import { ExecutableType } from '@/application/Parser/Executable/Validation/ExecutableType';
+import { CategoryDataStub } from './CategoryDataStub';
+
+export function createExecutableErrorContextStub(): ExecutableErrorContext {
+  return {
+    type: ExecutableType.Category,
+    self: new CategoryDataStub(),
+    parentCategory: new CategoryDataStub(),
+  };
+}