refactor: new version of MakeSingleFiles

[henryiii]

This makes "MakeSingleFiles" explicitly controlled by "magic comments". This provides enough freedom to work on the optional pre-compile mode, which otherwise would require significantly more advanced logic to pull off automatically when merging files - this should keep it manageable.

Comments look like this: // [CLI11:<name>:<action>], where action starts with verbatim or set, and ends with end. The names are then available to be used in a .in file. I've kept the script mostly general in design, though we have CLI11 specific defaults in it.

Maybe we should move the CLI11.hpp.in file somewhere other than the main dir?

[codecov]

Codecov Report

Merging #546 (24ddfb9) into master (822f3d6) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #546   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           12        12           
  Lines         3780      3780           
=========================================
  Hits          3780      3780           

Impacted Files	Coverage Δ
include/CLI/App.hpp	100.00% <ø> (ø)
include/CLI/Config.hpp	100.00% <ø> (ø)
include/CLI/ConfigFwd.hpp	100.00% <ø> (ø)
include/CLI/Error.hpp	100.00% <ø> (ø)
include/CLI/Formatter.hpp	100.00% <ø> (ø)
include/CLI/FormatterFwd.hpp	100.00% <ø> (ø)
include/CLI/Option.hpp	100.00% <ø> (ø)
include/CLI/Split.hpp	100.00% <ø> (ø)
include/CLI/StringTools.hpp	100.00% <ø> (ø)
include/CLI/TypeTools.hpp	100.00% <ø> (ø)
... and 1 more

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