implemented but disabled block-wise backwards search algorithm

* The block-wise search algorithm allows for efficient backwards searches
  on large files.
* On the downside the results are not entirely symmetric to forward searches.
  It therefore makes sense to still support the old correct but possibly slow
  algorithm.
  Since the old algorithm is just a special case of the new one (with a single
  block stretching the entire search range), you can configure the block size
  using `8EJ`.
* Unfortunately, the new block-wise algorithm won't work due to a bug in GRegex
  (only in the glib wrapper code).
  It is therefore disabled for the time being by default and will probably
  only be enabled once we switch to a new regexp engine.
  See https://gitlab.gnome.org/GNOME/glib/-/merge_requests/5199

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