Create a IsMapped expression

[hgraca]

This will allow us to ensure that certain classes are always in a map. This is useful, for example, to ensure that DTOs like commands and events are always set in a map, so that a serializer knows how to serialize/deserialize them.

[codecov-commenter]

Codecov Report

Merging #398 (3aee92f) into main (976c200) will increase coverage by 0.04%. The diff coverage is 100.00%.

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@@             Coverage Diff              @@
##               main     #398      +/-   ##
============================================
+ Coverage     94.33%   94.37%   +0.04%     
- Complexity      571      575       +4     
============================================
  Files            67       68       +1     
  Lines          1500     1511      +11     
============================================
+ Hits           1415     1426      +11     
  Misses           85       85              

Files Changed	Coverage Δ
src/Expression/ForClasses/IsMapped.php	100.00% <100.00%> (ø)

[AlessandroMinoccheri]

@hgraca Before approving, can you please add something to the readme file :) ?

[hgraca]

@AlessandroMinoccheri done