Re-name the RefSetCache class to RefMap

[Snuffleupagus]

The RefSet and RefSetCache classes are old enough that they predate a lot of modern JavaScript features, e.g. originally they stored their data in regular Objects.
About six years ago, see PRs #11977 and #12089, these classes were converted to use a Set respectively a Map internally.

Given that RefSet is a wrapper around a standard Set, it seems reasonable to re-name RefSetCache to RefMap when it's a wrapper around a standard Map.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.08%. Comparing base (022e958) to head (ed327b2).

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #21737      +/-   ##
==========================================
+ Coverage   90.05%   90.08%   +0.03%     
==========================================
  Files         264      264              
  Lines       67094    67093       -1     
==========================================
+ Hits        60422    60442      +20     
+ Misses       6672     6651      -21     

Flag	Coverage Δ
browsertest	66.32% <90.47%> (+0.02%)	⬆️
fonttest	9.03% <ø> (ø)
integrationtest	69.38% <80.95%> (+0.02%)	⬆️
unittest	58.34% <100.00%> (+<0.01%)	⬆️
unittestcli	56.70% <95.23%> (-0.02%)	⬇️

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[timvandermeij]

Good idea; thanks!