LibCore: Add support for double on argparse

Code is pretty trivial. If someone needs "float" support, a copy-paste
will be in place.

Build system was confused between math.h from rootfs, and toolchain. I
fixed the problem caused by `math.h` by locally using the builtin
`isnan()` from the compiler. It's ugly - but works. I am looking for
other alternatives.

Libraries/LibCore/ArgsParser.cpp

@@ -324,6 +324,28 @@ void ArgsParser::add_positional_argument(int& value, const char* help_string, co
     add_positional_argument(move(arg));
 }
 
+static constexpr bool isnan(double __x) { return __builtin_isnan(__x); }
+
+void ArgsParser::add_positional_argument(double& value, const char* help_string, const char* name, Required required)
+{
+    Arg arg {
+        help_string,
+        name,
+        required == Required::Yes ? 1 : 0,
+        1,
+        [&value](const char* s) {
+            char *p;
+            double v = strtod(s, &p);
+            bool valid_value = !isnan(v) && p != s;
+            if (valid_value) {
+                value = v;
+            }
+            return valid_value;
+        }
+    };
+    add_positional_argument(move(arg));
+}
+
 void ArgsParser::add_positional_argument(Vector<const char*>& values, const char* help_string, const char* name, Required required)
 {
     Arg arg {

Libraries/LibCore/ArgsParser.h

@@ -77,6 +77,7 @@ public:
     void add_positional_argument(Arg&&);
     void add_positional_argument(const char*& value, const char* help_string, const char* name, Required required = Required::Yes);
     void add_positional_argument(int& value, const char* help_string, const char* name, Required required = Required::Yes);
+    void add_positional_argument(double& value, const char* help_string, const char* name, Required required = Required::Yes);
     void add_positional_argument(Vector<const char*>& value, const char* help_string, const char* name, Required required = Required::Yes);
 
 private: