Implement GSC berry/apricorn tree functionality. (#7777)

Co-authored-by: Bassoonian <iasperbassoonian@gmail.com>

docs/tutorials/how_to_apricorn_tree.md

@@ -0,0 +1,58 @@
+# How to interact with Apricorn Trees
+
+![apricorn-tree](/docs/tutorials/img/apricorn_tree/apricorn-tree.gif)
+
+### Adding a new apricorn tree
+
+To add a new tree, first increase the tree count and expand the tree list in `include/constants/apricorn_tree.h`.
+
+Note that each tree will take a bit in the savegame's `SaveBlock3` struct so increasing `APRICORN_TREE_COUNT` **breaks the savegame**.
+Due to this, pokeemerald-expansion doesn't have any trees set up by default to prevent breaking downstream savegames.
+The trees support random yields and properly use plural case on plural yields.
+
+```diff
+#define APRICORN_TREE_NONE 0
+
+-#define APRICORN_TREE_COUNT 0
++#define APRICORN_TREE_ROUTE101_RED_TREE 1
++
++#define APRICORN_TREE_COUNT 32
+```
+
+Then list its data in `src/data/apricorns.h`.
+
+```diff
+const struct ApricornTree gApricornTrees[APRICORN_TREE_COUNT] =
+{
+    [APRICORN_TREE_NONE] =
+    {
+        .minimum = 1,
+        .maximum = 1,
+        .apricornType = APRICORN_RED,
+    },
+
++   [APRICORN_TREE_ROUTE101_RED_TREE] =
++   {
++       .minimum = 1,
++       .maximum = 1,
++       .apricornType = APRICORN_RED,
++   },
+};
+```
+Finally, just place your new tree using Porymap.
+Similarly to berries, the Sight Radius / Berry Tree ID field is used for the tree's ID.
+
+![apricorn-tree-porymap](/docs/tutorials/img/apricorn_tree/apricorn-tree-porymap.png)
+
+### Add a new apricorn type
+
+After you created your new item, simply expand the `ApricornType` enum in `include/constants/apricorn_tree.h`.
+
+```diff
+enum ApricornType
+{
+    [...]
+    APRICORN_BERRY_MARANGA = ITEM_MARANGA_BERRY,
++    APRICORN_BROWN         = ITEM_BROWN_APRICORN,
+};
+```