Add relations(::FPModule); redirect rels through relations

[lgoettgens]

Alternative to and thus closes #2168.
I'll provide the same kind of PR for Nemo, Hecke, and Oscar as well.

[lgoettgens]

We just noticed that there is @alias nrels number_of_relations in Hecke, so it would be most consistent to just merge #2168

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.01%. Comparing base (fd71cbf) to head (be04864).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/generic/InvariantFactorDecomposition.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2238   +/-   ##
=======================================
  Coverage   88.00%   88.01%           
=======================================
  Files         127      127           
  Lines       31798    31810   +12     
=======================================
+ Hits        27984    27996   +12     
  Misses       3814     3814           

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