BUG: Add support for PROJ_DATA environment variable

[snowman2]

• Closes REF: PROJ_DATA to replace PROJ_LIB environment variable #1097
• Tests added
• Fully documented, including history.rst for all changes and api/*.rst for new API

[codecov]

Codecov Report

Merging #1099 (58f96c8) into main (83ebc39) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1099   +/-   ##
=======================================
  Coverage   96.15%   96.15%           
=======================================
  Files          20       20           
  Lines        1794     1794           
=======================================
  Hits         1725     1725           
  Misses         69       69           

Impacted Files	Coverage Δ
pyproj/datadir.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 83ebc39...58f96c8. Read the comment docs.