Simplify number parsing (#386)

- use single test in `js_strtod` loop.
- use more explicit `ATOD_xxx` flags
- remove `ATOD_TYPE_MASK`, use `ATOD_WANT_BIG_INT` instead
- remove unused arguments `flags` and `pexponent` in `js_string_to_bigint`
- merge `js_atof` and `js_atof2`, remove `slimb_t *pexponent` argument
- simplify and document `js_atof` parser, remove cumbersome labels,
- simplify `js_parseInt` test for zero radix for `ATOD_ACCEPT_HEX_PREFIX`
- simplify `next_token` number parsing, handle legacy octal in parser only
- simplify `JS_StringToBigInt`, use flags only.
- remove unused `slimb_t exponent` token field
- add number syntax tests

Author: chqrlie