QuickJS: add missing btoa() and atob() and improve njs version

QuickJS: add missing btoa() and atob() and improve njs version

The functions implement the WHATWG btoa()/atob() global functions for
the QuickJS engine, mirroring the built-in njs engine.

Both engines decode forgiving-base64 in a single pass using a bit
accumulator, without allocating a scratch buffer for the
whitespace-stripped copy.

The shared behaviour is corrected and aligned with WHATWG:
- atob() strips all ASCII whitespace (TAB, LF, FF, CR, SPACE), not only
  SPACE; VT and NBSP are not ASCII whitespace and are rejected.
- btoa() error text says "(> U+00FF)" to match the cp > 0xff check.

The narrower njs-only btoa()/atob() unit tests are dropped in favor of
test/btoa.t.js, which exercises both engines.