Geometry Nodes: add Closures and Bundles behind experimental feature flag

This implements bundles and closures which are described in more detail in this
blog post: https://code.blender.org/2024/11/geometry-nodes-workshop-october-2024/

tl;dr:
* Bundles are containers that allow storing multiple socket values in a single
  value. Each value in the bundle is identified by a name. Bundles can be
  nested.
* Closures are functions that are created with the Closure Zone and can be
  evaluated with the Evaluate Closure node.

To use the patch, the `Bundle and Closure Nodes` experimental feature has to be
enabled. This is necessary, because these features are not fully done yet and
still need iterations to improve the workflow before they can be officially
released. These iterations are easier to do in `main` than in a separate branch
though. That's because this patch is quite large and somewhat prone to merge
conflicts. Also other work we want to do, depends on this.

This adds the following new nodes:
* Combine Bundle: can pack multiple values into one.
* Separate Bundle: extracts values from a bundle.
* Closure Zone: outputs a closure zone for use in the `Evaluate Closure` node.
* Evaluate Closure: evaluates the passed in closure.

Things that will be added soon after this lands:
* Fields in bundles and closures. The way this is done changes with #134811, so
  I rather implement this once both are in `main`.
* UI features for keeping sockets in sync (right now there are warnings only).

One bigger issue is the limited support for lazyness. For example, all inputs of
a Combine Bundle node will be evaluated, even if they are not all needed. The
same is true for all captured values of a closure. This is a deeper limitation
that needs to be resolved at some point. This will likely be done after an
initial version of this patch is done.

Pull Request: https://projects.blender.org/blender/blender/pulls/128340

release/datafiles/userdef/userdef_default_theme.c

@@ -901,6 +901,7 @@ const bTheme U_theme_default = {
     .node_zone_simulation = RGBA(0x66416233),
     .node_zone_repeat = RGBA(0x76512f33),
     .node_zone_foreach_geometry_element = RGBA(0x33527f33),
+    .node_zone_closure = RGBA(0x527F3333),
     .movie = RGBA(0x0f0f0fcc),
     .gp_vertex_size = 3,
     .gp_vertex = RGBA(0x97979700),