[libc++] Implement `std::ranges::merge`

Implement `std::ranges::merge`. added unit tests

Differential Revision: https://reviews.llvm.org/D128611

NOKEYCHECK=True
GitOrigin-RevId: 25607d143d1d98ac1b16ebeb6af4f3cceea0d738
diff --git a/include/module.modulemap.in b/include/module.modulemap.in
index a523579..becfec4 100644
--- a/include/module.modulemap.in
+++ b/include/module.modulemap.in
@@ -333,6 +333,7 @@
       module ranges_lower_bound              { private header "__algorithm/ranges_lower_bound.h" }
       module ranges_max                      { private header "__algorithm/ranges_max.h" }
       module ranges_max_element              { private header "__algorithm/ranges_max_element.h" }
+      module ranges_merge                    { private header "__algorithm/ranges_merge.h" }
       module ranges_min                      { private header "__algorithm/ranges_min.h" }
       module ranges_min_element              { private header "__algorithm/ranges_min_element.h" }
       module ranges_minmax                   { private header "__algorithm/ranges_minmax.h" }