Add some linting tools and fix one found problem

[hexmode]

Fixes #698 by using isset() instead of array_key_exists().

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This PR is made in reference to: #698

This PR addresses or contains:

• phpcs configuration
• update to composer.json to support common MW development

This PR includes:

[codecov-commenter]

Codecov Report

Merging #699 (b131ba8) into master (38bef6d) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #699   +/-   ##
=========================================
  Coverage     39.28%   39.28%           
  Complexity     2043     2043           
=========================================
  Files            75       75           
  Lines          7225     7225           
=========================================
  Hits           2838     2838           
  Misses         4387     4387           

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