Add Psalm workflow

[merkushin]

Resolves part of #7094

Proposed Changes

• Add Psalm workflow.

Testing Instructions

1. Create a branch from add/psalm-pipeline.
2. Push it to GitHub and create a PR.
3. Check Psalm Test check.

Pre-Merge Checklist

• PR title and description contain sufficient detail and accurately describe the changes
• Acceptance criteria is met
• Decisions are publicly documented
• Adheres to coding standards (PHP, JavaScript, CSS, HTML)
• All strings are translatable (without concatenation, handles plurals)
• Follows our naming conventions (P6rkRX-4oA-p2)
• Hooks (p6rkRX-1uS-p2) and functions are documented
• New UIs are responsive and use a mobile-first approach
• New UIs match the designs
• Different user privileges (admin, teacher, subscriber) are tested as appropriate
• Code is tested on the minimum supported PHP and WordPress versions
• User interface changes have been tested on the latest versions of Chrome, Firefox and Safari
• "Needs Documentation" label is added if this change requires updates to documentation
• Known issues are created as new GitHub issues

[codecov]

Codecov Report

Merging #7105 (1472cfc) into add/psalm-baseline (43181e8) will not change coverage.
The diff coverage is n/a.

@@                  Coverage Diff                  @@
##             add/psalm-baseline    #7105   +/-   ##
=====================================================
  Coverage                 49.35%   49.35%           
  Complexity                10541    10541           
=====================================================
  Files                       575      575           
  Lines                     44513    44513           
  Branches                    402      402           
=====================================================
  Hits                      21968    21968           
  Misses                    22218    22218           
  Partials                    327      327           

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[m1r0]

👍

The base branch was changed.