sen6x: Add driver for Sensirion SEN6x family of environmental sensors

[mtraver]

The Sensirion SEN6x family includes 6 sensors (SEN62, SEN63C, SEN65, SEN66, SEN68, SEN69C) that support the following measurements, in different combinations depending on the model:

• Particulate matter (PM1.0, PM2.5, PM4, and PM10, with the addition of PM0.5 from the Read Number Concentration Values command)
• Relative humidity
• Temperature
• VOC
• NOx
• Formaldehyde (HCHO)
• CO2

Datasheet: https://sensirion.com/media/documents/FAFC548D/693FBB15/PS_DS_SEN6x.pdf

[codecov]

Codecov Report

❌ Patch coverage is 97.30769% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 54.5%. Comparing base (1752d1b) to head (d375a1a).

Files with missing lines	Patch %	Lines
sen6x/sen6x.go	95.5%	8 Missing and 2 partials ⚠️
sen6x/vocnox.go	94.7%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #118     +/-   ##
=======================================
+ Coverage   52.6%   54.5%   +1.9%     
=======================================
  Files        117     127     +10     
  Lines      11801   12321    +520     
=======================================
+ Hits        6205    6713    +508     
- Misses      5232    5240      +8     
- Partials     364     368      +4     

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[mtraver]

The CI failures are all existing issues in dht22, inky, and lepton. Perhaps I'll put up PRs to fix those issues.

I know this is a giant PR... It covers a family of 6 sensors that support 7 different measurements, so there's quite a bit to implement. I've done my best to document it thoroughly and test it well (97% coverage currently, just a few lines that I found it hard to cover given how i2ctest works).

Functionality is broken up by measurement type, so there are different files for e.g. PM and CO2 functionality. The measurement-agnostic stuff and general I2C stuff is in sen6x.go.

Please let me know if I can assist during review!