Simplify JSAtom creation

- use single `JS_NewAtomRT` API to create atoms with or without `ctx`
  takes a UTF-8 string and flags, optionally detects numeric atoms
- simplify `__JS_NewAtom
- add `js_realloc_rt2` with slack handling
- simplify `JS_NewAtomLen` and `JS_NewAtom̀`
- simplify `JS_NewAtomUInt32`, `JS_NewAtomInt64`, `JS_NewClass`

Author: chqrlie