This fixes a bunch of FIXME's in LibLine.
Also handles the case where read() would read zero bytes in vt_dsr() and
effectively block forever by erroring out.
Fixes#2370
LibLine should ultimately not care about what a "token" means in the
context of its user, so force the user to split the buffer itself.
This also allows the users to pick up contextual clues as well, since
they have to lex the line themselves.
This commit pacthes Shell and the JS repl to better handle completions,
so certain wrong behaviours are now corrected as well:
- JS repl can now complete "Object . getOw<tab>"
- Shell can now complete "echo | ca<tab>" and paths inside strings