feat: reverse argument order of List.insert to index-first convention

The previous signature `insert(element, index)` was counter-intuitive and inconsistent with every major language's list API. This commit swaps the parameters to `insert(index, element)`, matching the convention used by Ruby, JavaScript, C++, Lua, C#, Java, and Python. All documentation, C implementation, and test files are updated to reflect the new order.

doc/site/core/list.markdown

@@ -17,7 +17,7 @@ Removes all items from the list.
 
 The number of items in the list.
 
-### **insert**(item, index)
+### **insert**(index, item)
 
 **TODO**
 

doc/site/lists.markdown

@@ -71,10 +71,10 @@ use `add` to append a single item to the end:
 You can insert a new element at a specific position using `insert`:
 
     :::dart
-    hirsute.insert("soul patch", 2)
+    hirsute.insert(2, "soul patch")
 
-The first argument is the value to insert, and the second is the index to
-insert it at. All elements following the inserted one will be pushed down to
+The first argument is the index to insert at, and the second is the value to
+insert. All elements following the inserted one will be pushed down to
 make room for it.
 
 It's valid to "insert" after the last element in the list, but only *right*
@@ -83,9 +83,9 @@ back. Doing so counts back from the size of the list *after* it's grown by one:
 
     :::dart
     var letters = ["a", "b", "c"]
-    letters.insert("d", 3)   // OK: inserts at end.
+    letters.insert(3, "d")   // OK: inserts at end.
     IO.print(letters)        // ["a", "b", "c", "d"]
-    letters.insert("e", -2)  // Counts back from size after insert.
+    letters.insert(-2, "e")  // Counts back from size after insert.
     IO.print(letters)        // ["a", "b", "c", "e", "d"]
 
 ## Removing elements