Runner: distribute files round-robin by size in parallel mode

[janedbal]

Description

Solves same problem as #1419 but with much simplier approach.

Measurements:

• before: 3 mins 43 secs
• after: 1 min 59 secs

(~22 400 PHP files, 24 workers)

Suggested changelog entry

• --parallel mode now better distributes work; gains up to 50% speedup

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
  • This change is only breaking for integrators, not for external standards or end-users.
• Documentation improvement

PR checklist

• I have checked there is no other PR open for the same change.
• I have read the Contribution Guidelines.
• I grant the project the right to include and distribute the code under the BSD-3-Clause license (and I have the right to grant these rights).
• I have added tests to cover my changes.
• I have verified that the code complies with the projects coding standards.
• [Required for new sniffs] I have added XML documentation for the sniff.
  • Not applicable
• I have opened a sister-PR in the documentation repository to update the Wiki.
  • I believe not needed