Refactoring for Gitlab::Diff::Highlight

Contributes to #16950 (closed)

The goal of this issue is to track related refactoring MRs.

• Extract PairSelector logic into a class - !55398 (merged)
• Update syntax highlighting logic - !56108 (merged)
• Introduce MarkerRanges class - !55669 (merged)
• Store MarkerRanges in Gitlab::Diff::Line objects - !56361 (merged)
• Detect MarkerRanges for word-diff mode - !56401 (merged)