Add a lid built-in expression variable

[abey79]

Description

This PR adds a lid built-in expression variable for generator and layer processor commands. This enables things like:

vpype [...] name "Layer %lid%" [...]

Checklist

• feature/fix implemented
• code formatting ok (black and isort)
• mypy returns no error
• tests added/updated and pytest succeeds
• documentation added/updated
  • command docstring and option/argument help
  • README.md updated (Feature Overview)
  • CHANGELOG.md updated
  • added new command to reference.rst
  • RTD doc updated and building with no error (make clean && make html in docs/)

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (b70ab77) 94.73% compared to head (c973f25) 94.74%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #605   +/-   ##
=======================================
  Coverage   94.73%   94.74%           
=======================================
  Files          62       62           
  Lines        5683     5690    +7     
  Branches     1300     1302    +2     
=======================================
+ Hits         5384     5391    +7     
  Misses        182      182           
  Partials      117      117           

Impacted Files	Coverage Δ
tests/test_substitution.py	98.40% <100.00%> (+0.06%)	⬆️
vpype_cli/decorators.py	95.34% <100.00%> (+0.11%)	⬆️

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