LibCore: Make ArgParser::Arg::accept_value return ErrorOr<bool>

Much like the previous commit, this commit makes the
ArgParser::Arg::accept_value callback return an ErrorOr<bool> instead of
just a bool.

The aim of this is to make argument parsing more robust, especially
with the newer String api that returns an ErrorOr for many functions.

Userland/Libraries/LibCore/ArgsParser.h

@@ -67,7 +67,7 @@ public:
         char const* name { nullptr };
         int min_values { 0 };
         int max_values { 1 };
-        Function<bool(StringView)> accept_value;
+        Function<ErrorOr<bool>(StringView)> accept_value;
     };
 
     bool parse(Span<StringView> arguments, FailureBehavior failure_behavior = FailureBehavior::PrintUsageAndExit);