build: bump to C++20 (#7191)

* build: bump to C++20

Co-authored-by: Cœur <coeur@gmx.fr>

* refactor: use designated initializers

* refactor: remove redundant SFINAE

* fix: clang-tidy warnings

* chore: comments about min compiler versions for C++20 features

* build: clang objc++ modules build errors

Co-authored-by: Dzmitry Neviadomski <nevack.d@gmail.com>

* refactor: add `TR_CONSTEXPR_VEC` and `TR_CONSTEXPR_STR`

* fix: don't use `std::rel_ops`

* chore: housekeeping

* fix: possible fix for macOS linker error

---------

Co-authored-by: Cœur <coeur@gmx.fr>
Co-authored-by: Dzmitry Neviadomski <nevack.d@gmail.com>
Co-authored-by: Charles Kerr <charles@charleskerr.com>

macosx/QuickLookPlugin/GeneratePreviewForURL.mm

@@ -1,6 +1,12 @@
+#if __has_feature(modules)
 @import AppKit;
 @import CoreFoundation;
 @import QuickLook;
+#else
+#import <AppKit/AppKit.h>
+#import <CoreFoundation/CFPlugInCOM.h>
+#import <QuickLook/QuickLook.h>
+#endif
 
 #include <string>
 #include <unordered_map>