deps: update acorn to 6.0.7

acorn and acorn-walk are now published as two different packages. Put
them both in subdirectories of `deps/acorn`.
Adapt the REPL's recoverable error detection to use the new API for
extending acorn parsers.

PR-URL: #25844
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

Author: targos