AK+Userland: Extend the compiletime format string check to other functions

Thanks to @trflynn89 for the neat implicit consteval ctor trick!
This allows us to basically slap `CheckedFormatString` on any
formatting function, and have its format argument checked at compiletime.

Note that there is a validator bug where it doesn't parse inner replaced
fields like `{:~>{}}` correctly (what should be 'left align with next
argument as size' is parsed as `{:~>{` following a literal closing
brace), so the compiletime checks are disabled on these temporarily by
forcing them to be StringViews.

This commit also removes the now unused `AK::StringLiteral` type (which
was introduced for use with NTTP strings).

AK/TestSuite.h

@@ -27,11 +27,12 @@
 #pragma once
 
 #include <AK/Assertions.h>
+#include <AK/CheckedFormatString.h>
 
 namespace AK {
 
 template<typename... Parameters>
-void warnln(const char* fmtstr, const Parameters&...);
+void warnln(CheckedFormatString<Parameters...>&& fmtstr, const Parameters&...);
 
 }