DialogueRunner.VariableStorage changed to interface instead of MonoBehaviour class.

[Houtamelo]

• [X] Tests for the changes have been added (for bug fixes / features)
  • [X] Does it pass all existing unit tests without modification?
• [X] Docs have been added / updated (for bug fixes / features)
• [X] CHANGELOG.md has been updated to describe this change

• What kind of change does this pull request introduce?

• [X] Feature

• What is the current behavior?

DialogueRunner.VariableStorage is a MonoBehaviour which doesn't allow users to setup their own implementation of the base class without having a MonoBehaviour as the intermediary.

• What is the new behavior (if this is a feature change)?

DialogueRunner.VariableStorage is now an interface called IExtendedVariableStorage.

• Does this pull request introduce a breaking change?

No.

• Who will be affected?
  • Is this an internal change, or something meant to be consumed by the end user? A: Meant for the end user.
  • If you add API changes, is it easily upgradable from the previous version? A: Yes.
• What do you think will be controversial, if any? A: The implementation of the DialogueRunner.VariableStorage property.
• How would you describe the cause of the problem and changes to non-technical users if at all possible? A: Non-technical users won't be affected as the previous functionatily still works in the same manner.