Variables: Fix support for "${self:}"

[medikoo]

It was not working for many commands, not long after introduction with #3208 (published with v1.9.0) - due to not proper handling of circular references (fixed in this PR)

Internal recognition for some commands was additionally wiped out with #8279 (published with v2.3.0) - due to fine tuning of variables regex (fixed in this PR)

This brings it back, although this functionality feels controversial, and we may consider to deprecate it at some point in a future.

[codecov-commenter]

Codecov Report

Merging #8343 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #8343      +/-   ##
==========================================
- Coverage   88.01%   88.01%   -0.01%     
==========================================
  Files         249      249              
  Lines        9306     9305       -1     
==========================================
- Hits         8191     8190       -1     
  Misses       1115     1115              

Impacted Files	Coverage Δ
lib/classes/Service.js	85.48% <ø> (ø)
lib/classes/Variables.js	99.72% <100.00%> (-0.01%)	⬇️

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