[precompiled] Initial proof-of-concept with App.cpp

[dherrera-fb]

This is a proof-of-concept PR to show a potential implementation of https://github.com/CLIUtils/CLI11/issues/338

• Add C11_COMPILE cmake option that creates a static lib instead of header-only
• Add C11_INLINE macro that depends on C11_COMPILE
• Split App.hpp into App.hpp and impl/App_inl.hpp
• Add App.cpp that compiles App_inl.hpp into an object file
• CMake modifications to handle impl headers differently for sinlge-header, headers-only, and compiled versions

[codecov[bot]]

Codecov Report

Merging #759 (0a1e08c) into main (c57000e) will decrease coverage by 0.00%. The diff coverage is 99.19%.

:exclamation: Current head 0a1e08c differs from pull request most recent head ccf7abe. Consider uploading reports for the commit ccf7abe to get more accurate results

@@            Coverage Diff             @@
##             main     #759      +/-   ##
==========================================
- Coverage   99.46%   99.46%   -0.01%     
==========================================
  Files          12       13       +1     
  Lines        3958     3947      -11     
==========================================
- Hits         3937     3926      -11     
  Misses         21       21              

Impacted Files	Coverage Δ
include/CLI/App.hpp	100.00% <ø> (+0.64%)	:arrow_up:
include/CLI/impl/App_inl.hpp	99.19% <99.19%> (ø)

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