Write node groups on a single line when saving a .tscn file

[Calinou]

Split from #32124.

This makes .tscn files more readable by ensuring sections are always written on a single line. This is compatible with existing scenes (both backwards and forwards-compatible).

As a bonus, it also results in better syntax highlighting on GitHub when using the ini syntax highlighter 🙂

Preview

Before

[gd_scene format=3 uid="uid://dctlrgcoo4tc8"]

[node name="Node2D" type="Node2D" groups=[
"2",
"bar",
"baz",
"foo",
"hello",
"world",
]]
script = null

[node name="Sprite2D" type="Sprite2D" parent="." groups=[
"hello",
]]
position = Vector2(15, 18)
script = null

[node name="Sprite2D2" type="Sprite2D" parent="."]
script = null

After

[gd_scene format=3 uid="uid://dctlrgcoo4tc8"]

[node name="Node2D" type="Node2D" groups=["2", "bar", "baz", "foo", "hello", "world"]]
script = null

[node name="Sprite2D" type="Sprite2D" parent="." groups=["hello"]]
position = Vector2(15, 18)
script = null

[node name="Sprite2D2" type="Sprite2D" parent="."]
script = null

[akien-mga]

Cherry-picked for 3.4.