Fix a build failure on Apple Clang 17.0.0 related to the distance calculation in js_string_convert_pos

[GHamrouni]

The code previously called abs() on the result of an unsigned subtraction (uint32_t).
While the code relied on underflow and implicit casting to int to function, Clang flags this as having no effect (or undefined behavior).

Compiler Error:

gcc -Wall -g -MMD -Werror -D_GNU_SOURCE -fno-math-errno -fno-trapping-math -Os -c -o mquickjs.o mquickjs.c
mquickjs.c:1476:17: error: taking the absolute value of unsigned type 'uint32_t' (aka 'unsigned int') has no effect [-Werror,-Wabsolute-value]
 1476 |             d = abs(ce1->str_pos[pos_type] - pos);
      |                 ^
mquickjs.c:1476:17: note: remove the call to 'abs' since unsigned values cannot be negative
 1476 |             d = abs(ce1->str_pos[pos_type] - pos);
      |                 ^~~
1 error generated.
make: *** [mquickjs.o] Error 1

Resolution: Replaced the abs() call with a manual conditional subtraction to correctly calculate the distance between the two unsigned positions without relying on implicit casts or underflow.

[bellard]

already fixed