Add NUMERIC compare macros to simplify code

Marlin/Marlin_main.cpp

   while (c == ' ') c = seen_pointer[++i];
   if (c == '-' || c == '+') c = seen_pointer[++i];
   if (c == '.') c = seen_pointer[++i];
-  return (c >= '0' && c <= '9');
+  return NUMERIC(c);
 }
 
 float code_value() {
   //  - Bypass N[-0-9][0-9]*[ ]*
   //  - Overwrite * with nul to mark the end
   while (*current_command == ' ') ++current_command;
-  if (*current_command == 'N' && ((current_command[1] >= '0' && current_command[1] <= '9') || current_command[1] == '-')) {
+  if (*current_command == 'N' && NUMERIC_SIGNED(current_command[1])) {
     current_command += 2; // skip N[-0-9]
-    while (*current_command >= '0' && *current_command <= '9') ++current_command; // skip [0-9]*
+    while (NUMERIC(*current_command)) ++current_command; // skip [0-9]*
     while (*current_command == ' ') ++current_command; // skip [ ]*
   }
   char* starpos = strchr(current_command, '*');  // * should always be the last parameter
     if (*p == 'N') {
       SERIAL_PROTOCOL(' ');
       SERIAL_ECHO(*p++);
-      while ((*p >= '0' && *p <= '9') || *p == '-')
+      while (NUMERIC_SIGNED(*p))
         SERIAL_ECHO(*p++);
     }
     SERIAL_PROTOCOLPGM(" P"); SERIAL_PROTOCOL(int(BLOCK_BUFFER_SIZE - movesplanned() - 1));

Marlin/macros.h

 #define ENABLED(b) _CAT(SWITCH_ENABLED_, b)
 #define DISABLED(b) (!_CAT(SWITCH_ENABLED_, b))
 
+#define NUMERIC(a) ((a) >= '0' && '9' >= (a))
+#define NUMERIC_SIGNED(a) (NUMERIC(a) || (a) == '-')
 #define COUNT(a) (sizeof(a)/sizeof(*a))
 
 #endif //__MACROS_H