Simplify number parsing

- use single test in `js_strtod` loop.
- use more explicit ATOD_xxx flags
- remove unused arguments `flags` and `pexponent` in `js_string_to_bigint`
- merge `js_atof` and `js_atof2`, remove `slimb_t *pexponent` argument
- simplify `js_atof` parser, remove cumbersome labels,
- remove unused `slimb_t exponent` token field

Author: chqrlie