added a function that can filter error trees which are in optional positions in a grammar (list elements and optionals)

[jurgenvinju]

This could be one of those library functions in Recovery.rsc that is useful to call from a language service. It uses information from grammar rules (so "language-parametric") to decide that the error tree can be ignored in principle.

• list elements that are erroneous are simply removed while the other elements remain, including surrounding separators.
• optional elements which happen to be errors are simply removed.

have to add tests for this of course, before we merge.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 49%. Comparing base (415773f) to head (2725c65).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## feat/error-recovery #2074 +/- ## ===================================================== Coverage 49% 49% Complexity 6605 6605 ===================================================== Files 685 685 Lines 61148 61148 Branches 8850 8850 ===================================================== + Hits 30239 30243 +4 + Misses 28706 28702 -4 Partials 2203 2203 ```

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