Diffs: Default values innacurate

Default Vector2 values display as `(0, 0)` instead of `Vector2(0, 0)`:

Default values on an instantiated root node from a different scene are missing entirely:

Start by locating the diff rendering or default-value handling for Godot scenes, then reproduce both cases shown in the issue: a default Vector2 and an instantiated root node from another scene. Done means the Vector2 display includes its type and the instantiated root node's default values are shown accurately in diffs.

Written by the indexing model from the issue text.