Mark train stations and factories as experimental (#1309)

## Description:

Train stations and factories won't be ready for v24.
Disable them by default. They can be reactivated on private lobies and
solo games, allowing us to gather feedbacks.

Changes:
- added an "experimental" attribute for units. When set, they are hidden
entirely when disabled, rather than appearing grayed out.
 - disabled train stations and factories by default.

Default values:

![image](https://github.com/user-attachments/assets/ec91ef80-c0ef-45a7-8e6c-6628a836b593)

No factories:

![image](https://github.com/user-attachments/assets/a3e35607-aed5-401c-bc38-4ea742d057c6)


## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced
- [x] I understand that submitting code with bugs that could have been
caught through manual testing blocks releases and new features for all
contributors

## Please put your Discord username so you can be contacted if a bug or
regression is found:

IngloriousTom

src/client/SinglePlayerModal.ts

@@ -40,7 +40,10 @@ export class SinglePlayerModal extends LitElement {
   @state() private gameMode: GameMode = GameMode.FFA;
   @state() private teamCount: number | typeof Duos = 2;
 
-  @state() private disabledUnits: UnitType[] = [];
+  @state() private disabledUnits: UnitType[] = [
+    UnitType.Factory,
+    UnitType.Train,
+  ];
 
   private userSettings: UserSettings = new UserSettings();