[libc++][format] Adds parser std-format-spec.

This implements the generic std.format.spec framework for all types.

The Unicode support will be added in a separate patch.

Implements parts of:
- P0645 Text Formatting

Completes:
- LWG-3242 std::format: missing rules for arg-id in width and precision
- P1892 Extended locale-specific presentation specifiers for std::format

Reviewed By: #libc, ldionne, vitaut

Differential Revision: https://reviews.llvm.org/D103368

NOKEYCHECK=True
GitOrigin-RevId: a04a6ce7726b51c4f503c8de899362bee40d4e04
diff --git a/include/format b/include/format
index 25350f7..dcedc32 100644
--- a/include/format
+++ b/include/format
@@ -277,6 +277,7 @@
 #include <__format/format_error.h>
 #include <__format/format_parse_context.h>
 #include <__format/format_string.h>
+#include <__format/parser_std_format_spec.h>
 #include <__format/formatter.h>
 #include <__variant/monostate.h>
 #include <array>