Change to Rust 2024 edition and remove unsafe environment var mutation

[jtran]

See each commit separately.

Environment variable mutation became unsafe in Rust 2024. Instead of using serial_test to run everything single-threaded or hacking around it and praying that we use it safely, I decided to remove it.

Some tests became unit tests (colors.rs). Some tests didn't actually need the environment variables set (cmd_user.rs). Some tests could have their variables injected (context.rs). But presumably we may need this in the future since kcl_lib may try to read environment variables, so I made kcl_process.rs that creates a child process where we can control its environment variables without mutating the current process's env.

Hopefully, in the future, we can remove serial_test. It's still used in a lot of places, but I didn't want to expand scope more.

[codecov]

Codecov Report

❌ Patch coverage is 59.66851% with 73 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.28%. Comparing base (48786c0) to head (11b3bfe).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/context.rs	52.21%	54 Missing ⚠️
src/cmd_ml/cmd_text_to_cad.rs	0.00%	6 Missing ⚠️
src/ml/copilot/run.rs	0.00%	4 Missing ⚠️
src/colors.rs	85.71%	3 Missing ⚠️
src/main.rs	0.00%	3 Missing ⚠️
src/cmd_api_call.rs	0.00%	2 Missing ⚠️
src/cmd_kcl.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1712      +/-   ##
==========================================
+ Coverage   65.21%   66.28%   +1.06%     
==========================================
  Files          44       44              
  Lines        8219     8476     +257     
==========================================
+ Hits         5360     5618     +258     
+ Misses       2859     2858       -1     

Flag	Coverage Δ
unittests	66.28% <59.66%> (+1.06%)	⬆️

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