feat(atoll): SKY130 ATOLL plugin NMOS tile generator

[rahulk29]

• feat(atoll): support sky130 atoll-compatible nmos tiles
• export nmos tile schematic in test
• fix(docs): add doc comments for NmosTile
• write out spectre netlist in sky130 atoll nmos tile test

[codecov]

Codecov Report

Attention: 80 lines in your changes are missing coverage. Please review.

Comparison is base (c95e5c0) 39.30% compared to head (a0380c9) 39.44%.

Files	Patch %	Lines
pdks/sky130pdk/src/atoll/mod.rs	52.38%	11 Missing and 59 partials ⚠️
tests/src/atoll/mod.rs	40.00%	0 Missing and 9 partials ⚠️
pdks/sky130pdk/src/lib.rs	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #350      +/-   ##
==========================================
+ Coverage   39.30%   39.44%   +0.13%     
==========================================
  Files         162      163       +1     
  Lines       20068    20214     +146     
  Branches     8406     8472      +66     
==========================================
+ Hits         7888     7973      +85     
+ Misses       6709     6664      -45     
- Partials     5471     5577     +106     

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