parser.c: Optimize json_parse_digits

We can use `ctz` builtin to get the number of consecutive digits.

```
== Parsing float parsing (2251051 bytes)
ruby 3.4.6 (2025-09-16 revision dbd83256b1) +YJIT +PRISM [arm64-darwin24]
Warming up --------------------------------------
               after    25.000 i/100ms
Calculating -------------------------------------
               after    231.963 (± 0.0%) i/s    (4.31 ms/i) -      1.175k in   5.065467s

Comparison:
              before:      215.3 i/s
               after:      232.0 i/s - 1.08x  faster
```

Author: byroot

ext/json/ext/parser/parser.c

@@ -1101,13 +1101,20 @@ static inline int json_parse_digits(JSON_ParserState *state, uint64_t *accumulat
             continue;
         }
 
-        if ((match & 0xFFFFFFFF) == 0x33333333) { // 4 consecutive digits
+        size_t consecutive_digits = trailing_zeros64(match ^ 0x3333333333333333) / CHAR_BIT;
+        if (consecutive_digits >= 4) {
             *accumulator = (*accumulator * 10000) + decode_4digits_unrolled((uint32_t)next_8bytes);
             state->cursor += 4;
-            break;
+            consecutive_digits -= 4;
+        }
+
+        while (consecutive_digits) {
+            *accumulator = *accumulator * 10 + (*state->cursor - '0');
+            consecutive_digits--;
+            state->cursor++;
         }
 
-        break;
+        return (int)(state->cursor - start);
     }
 #endif