Add deprecated version

Author: lpil

CHANGELOG.md

@@ -3,7 +3,8 @@
 ## Unreleased
 
 - The `list` module gains the `list.map2` function.
-- `flatten` has been renamed to `concat` in the `list` module.
+- `flatten` has been renamed to `concat` in the `list` module. The old name is
+  still available as an alias and is deprecated.
 
 ## v0.29.2 - 2023-06-21
 

src/gleam/list.gleam

@@ -683,6 +683,12 @@ pub fn concat(lists: List(List(a))) -> List(a) {
   do_concat(lists, [])
 }
 
+// TODO: Add deprecation attribute and then remove later.
+/// This function is deprecated, see `concat` instead.
+pub fn flatten(lists: List(List(a))) -> List(a) {
+  do_concat(lists, [])
+}
+
 /// Maps the list with the given function into a list of lists, and then flattens it.
 ///
 /// ## Examples