Fix header scanning regex

[tcbrindle]

...to allow us to pick up headers with extension .h as well as .hpp when building the single header

Fixes #221

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.60%. Comparing base (9a02b77) to head (76ecd19).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #222   +/-   ##
=======================================
  Coverage   98.60%   98.60%           
=======================================
  Files          69       69           
  Lines        2578     2578           
=======================================
  Hits         2542     2542           
  Misses         36       36           

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