Fix identifying nested reference placeholders

[varjolintu]

Deeply nested reference placeholders were not properly detected. Those must not be bound to the ResolveMaximumDepth.

Moved matchReference() to EntryPlaceholders class, and used that to check for nested reference placeholders if the placeholder type was not known.

Testing strategy

Automatic tests added.

Type of change

• ✅ Bug fix (non-breaking change that fixes an issue)

[codecov]

Codecov Report

❌ Patch coverage is 94.11765% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 63.13%. Comparing base (5ac1b0c) to head (a9c3b7e).

Files with missing lines	Patch %	Lines
src/core/Entry.cpp	66.67%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop   #13688   +/-   ##
========================================
  Coverage    63.13%   63.13%           
========================================
  Files          388      388           
  Lines        41129    41131    +2     
========================================
+ Hits         25964    25966    +2     
  Misses       15165    15165           

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