refactor(core): added a PopulatePath enum, to contain the special prop names in populate

[boenrobot]

Currently, this includes just "*" and "$infer".
Places where "*" is used as an actual SQL query expression remain using the literal string "*".

[boenrobot]

If this gets in, I can extend #5359 to resolve conflicts with any prop that for whatever reason the schema has defined as "*" or "$infer", or whatever else MikroORM may add in the future as a special value in "populate", and do so in a future proof fashion.

Site note: I'm not 100% why in some spots, specifying PopulatePath.ALL works, while others require `${PopulatePath.ALL}`... I tried to use PopulatePath.ALL everywhere where "*" is allowed while "$infer" is not, but that doesn't quite work always somehow. The lib builds even without these, but the tests don't. But since the aim is 0 BC breaks, I guess that should be OK.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.69%. Comparing base (e32ae32) to head (40e98d5).

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #5418    +/-   ##
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  Coverage   99.69%   99.69%            
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  Files         238      238            
  Lines       17251    17254     +3     
  Branches     4003     3702   -301     
========================================
+ Hits        17199    17202     +3     
- Misses         51       52     +1     
+ Partials        1        0     -1     

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