Improve shell script detection based on emacs file mode header 🐯

[jamacku]

• add support for headers with mode: - e.g. -*- mode: sh -*-
• add support for headers with shell script key

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.10%. Comparing base (5580924) to head (2cab78d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #357   +/-   ##
=======================================
  Coverage   85.10%   85.10%           
=======================================
  Files           4        4           
  Lines         282      282           
=======================================
  Hits          240      240           
  Misses         42       42           

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