Add ability to get the line range for a multi-line comment

What does this MR do and why?

This is part 1 of a 3-part change. This introduces the underlying logic to provide the set of lines needed for full front end comprehension of a multi-line comment.

Part	Purpose	MR
1	Add diffs store action implementation	👈🏻 You are here
2	Pass comment lines range to the reply box	!202464 (merged)
3	Pass comment lines range to the comment edit interface	!202711 (merged)

References

• For #557266 (closed)

Screenshots or screen recordings

N/A, all backstage

To see how this will be used, see either part 2 or 3, which update the UI.

MR acceptance checklist

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