Introduce MarkerRange class [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Contributes to #16950 (closed)

MarkerRange behaves like a Range but can additionally keep information about the type of the change.

It helps to identify if changed characters were removed or added.

Why?

1. Information about marker diff ranges does not belong to Gitlab::Diff::Line and calculated separately. Every time we need to mark a range, we have to compare all related diff lines, find changed characters and render the result. After this point range data is lost because we don't associate it with a diff line. I think it would be better if we keep it in Gitlab::Diff::Line.

2. Our current solution is based on the assumption that Gitlab::Diff::Line can only have either added or removed characters. But it won't work for the word-diff presentation when a single diff line can have both markers.

MarkerRanges will allow us to define complex marker definitions. See an example below.

Gitlab::Diff::Line.new(..., marker_ranges: [
  MarkerRange.new(0, 10, mode: :addition), 
  MarkerRange.new(15, 17, mode: :addition), 
  MarkerRange.new(20, 30, mode: :deletion)
])

Screenshots (strongly suggested)

Word-diff view prototype

Does this MR meet the acceptance criteria?

Conformity

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  • I have not included a changelog entry because the feature is behind a feature flag #324068.
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