[libc++] Implement format_error.

This is the first step at implementing <format>. It adds the <format> header
and implements the `format_error`. class.

Implemnts parts of:
-P0645 Text Formatting

Reviewed By: ldionne, #libc, miscco, curdeius

Differential Revision: https://reviews.llvm.org/D92214

GitOrigin-RevId: 081c1db02dd2b48cf736d44862f7d7e8c2869577
diff --git a/include/module.modulemap b/include/module.modulemap
index 750cd38..56b49c4 100644
--- a/include/module.modulemap
+++ b/include/module.modulemap
@@ -292,6 +292,10 @@
     header "filesystem"
     export *
   }
+  module format {
+    header "format"
+    export *
+  }
   module forward_list {
     header "forward_list"
     export initializer_list