Refactor the Add menu in Texture Nodes, with manually created menus,
inspired by Geometry Nodes and more recently Shader and Compositor.
Minor sorting adjustments by splitting categories in groups, with
separators in between groups, and sorted alphabetically.
Unlike other node editors, this menu will not feature assets for the time
being. Doing so would add (more) burden of maintenance to a system
that is deemed end-of-life, and likely to be rewritten before long.
Part of #111746
Pull Request: https://projects.blender.org/blender/blender/pulls/111838
5fac8f822b