Add support for [ul] tags in documentation

[RedMser]

Previously, unordered lists were only rendering properly in RST format (RST defines them as any line starting with - or * or +). In Godot's documentation viewer and rich tooltips, they showed up as plain text.

This PR adds support for the [ul] tag in documentation, which is how unordered lists are declared in RichTextLabel:

**OLD SYNTAX**
- First
- Second

**NEW SYNTAX**
[ul]
First
Second
[/ul]

Of course, RST support had to be added back as well, by converting [ul] tags back to old syntax.

The alternative solution would be to keep old syntax, and to convert it to [ul] lists for EditorHelp, as is done in #88900

In a second commit, the new tag is used everywhere in the class docs. I'll resolve conflicts once code review is done.

---

I don't have the RST -> HTML build setup, so I can't verify the result of the following edge case in the conversion:

- :ref:`Node._shortcut_input<class_Node_method__shortcut_input>`\ 
- :ref:`Node._unhandled_input<class_Node_method__unhandled_input>`\ 
- :ref:`Node._unhandled_key_input<class_Node_method__unhandled_key_input>`\ 

where each line ends with <backslash><space>.

• Bugsquad edit, closes: Unordered/ordered list tags don't work in auto-created documentation #84798

[RedMser]

Once I get to rebase, I can look into supporting [ol] as well. Ordered lists are already used in these files:

PhysicsServer2D, PhysicsServer3D, SceneTree and there are some "in the following order" lists which should likely use numbers as well in Viewport, Node.

But work on this PR also depends on whether #88900 's approach is preferred.