A pure read lists the pairs of open pulls that touch the same files, most files shared first

Part of #5930: every pair is a conflict waiting for the first to land.

Done:

[ ] New src/flow/conflict_pairs.ts exports conflictPairs(pulls), taking { number, files }[] and returning { a, b, shared }[] (a < b), most shared first, then lowest numbers; at most 20.
[ ] Pulls touching more than 60 files are left out, as the hot-file limit does for wide pulls.
[ ] Tests: no overlap, one pair, ordering, the wide-pull rule, the cap.

Files: src/flow/conflict_pairs.ts, test/flow-conflict-pairs.test.ts.

Before pushing: bun x tsc --noEmit and the new test.

A pure read lists the pairs of open pulls that touch the same files, most files shared first #6549