Support for parsing input that forms a valid prefix

[PieterOlivier]

Previously, if an input string was a valid prefix parsing would fail. This was often the case after error recovery, especially in Rascal files.
This PR adds support for recovering from such a parse error. When this situation is encountered, an error node is generated that skips zero characters at the end of the file.

This PR also adds support in the Rascal error recovery api to access multiple error nodes after disambiguation.

It also refactores and extends the test code.

[codecov]

Codecov Report

Attention: Patch coverage is 39.62264% with 32 lines in your changes missing coverage. Please review.

Project coverage is 49%. Comparing base (bdee4f4) to head (10ce970).
Report is 11 commits behind head on feat/error-recovery.

Files with missing lines	Patch %	Lines
...rg/rascalmpl/parser/util/ParseStateVisualizer.java	0%	27 Missing ⚠️
src/org/rascalmpl/parser/util/DebugUtil.java	0%	4 Missing ⚠️
src/org/rascalmpl/parser/gtd/SGTDBF.java	93%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@                  Coverage Diff                  @@
##             feat/error-recovery   #2053   +/-   ##
=====================================================
  Coverage                     49%     49%           
- Complexity                  6581    6582    +1     
=====================================================
  Files                        685     685           
  Lines                      61102   61127   +25     
  Branches                    8835    8840    +5     
=====================================================
+ Hits                       30174   30197   +23     
- Misses                     28715   28723    +8     
+ Partials                    2213    2207    -6     

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[DavyLandman]

LGTM