feat: forbid data variables with the same name on instance

[TrySound]

Ref #4768

We don't want to allow data variables on one instance with the same name. Though we will allow child variables to mask parent ones in another PR.

To test:

• create variable var1 on box
• try to create another variable with var1
• create variable var2
• try to rename it to var1
• create variable on body with var1

[kof]

What happens with the existing variables which have the same name?

[TrySound]

Nothing, users will get the error when try to edit them. Though I doubt anybody name variables the same.