Feature: Add environment variable parsing.

[BrianPugh]

Allows for environment variable parsing fallbacks:

@app.default
def foo(bar: Annotated[int, Parameter(env_var="BAR")] = 123):
    pass

The priority order is:

1. CLI-provided values
2. Values parsed from Parameter.env_var.
3. Default values from the function signature.

Addresses #18.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (bcabd8f) 92.50% compared to head (537b428) 92.64%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #20      +/-   ##
==========================================
+ Coverage   92.50%   92.64%   +0.14%     
==========================================
  Files          10       10              
  Lines        1080     1102      +22     
  Branches      258      262       +4     
==========================================
+ Hits          999     1021      +22     
  Misses         45       45              
  Partials       36       36              

Flag	Coverage Δ
unittests	92.55% <100.00%> (+0.15%)	⬆️

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[BrianPugh]

@rbonthond let me know what you think of this.

[rbonthond]

that works for me, thank you!