fix: loader conditionals

[maxb2]

• Adds the ability to conditionally execute the underlying loader via typer_config.loaders.loader_transformer. Alternatively, you can define your own loader with a conditional inside.
• Makes the provided callbacks (typer_config.*_conf_callback) not call the loader by default when the parameter value is empty (fixes --help behavior).
• Adds tests to make sure --help doesn't crash.

Fixes #24

[codecov]

Codecov Report

Merging #23 (8b7e6eb) into main (edadf7c) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #23   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines           98       102    +4     
=========================================
+ Hits            98       102    +4     

Impacted Files	Coverage Δ
typer_config/__init__.py	100.00% <100.00%> (ø)
typer_config/__typing.py	100.00% <100.00%> (ø)
typer_config/loaders.py	100.00% <100.00%> (ø)

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