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).

Userland/Utilities/expr.cpp

@@ -45,11 +45,11 @@ Print the value of EXPRESSION to standard output.)");
     exit(0);
 }
 
-template<typename... Args>
-[[noreturn]] void fail(Args&&... args)
+template<typename Fmt, typename... Args>
+[[noreturn]] void fail(Fmt&& fmt, Args&&... args)
 {
     warn("ERROR: \e[31m");
-    warnln(args...);
+    warnln(StringView { fmt }, args...);
     warn("\e[0m");
     exit(1);
 }