Update Godot Dialogue Manager addon

[wjt]

We currently have v3.7.1 but the latest at the time of writing is v3.9.1.

Reviewing the changelog, nathanhoad/godot_dialogue_manager#1002 looks interesting - I think it would achieve part of #1346 for free.

I am also interested in nathanhoad/godot_dialogue_manager#988 which would be extremely useful if someone wanted to add voiceover.

Generally the steps to update this addon are as follows:

1. Check whether we have any local changes to the addon (git log addons/dialogue_manager/). If so, stop and investigate whether these are changes we want to keep. Only continue if there are no important changes.
2. In the Dialogue tab in the editor, click the "v3.9.1 available" button, click "Download update"
3. Commit the result
4. Play the game a little, going through some scenes with interesting dialogue (e.g. the musician uses branching dialogue and references other nodes in the scene) to make sure nothing is broken.
5. (If things are broken, fix them and commit the result; or give up and document what broke on this issue.)
6. Open a PR!

Any further changes to use new features should be made in separate PRs.

XP Summary (total 175)

Subdomain	Skill	XP
Engineering: Gameplay & Systems Programming	Problem Solving	50
Engineering: Gameplay & Systems Programming	Creativity & Design Thinking	50
Engineering: Gameplay & Systems Programming	Technical Literacy	75