Reduce how much text needs to be diffed when computing cursor positions (#15709)

Co-authored-by: fisker Cheung <lionkay@gmail.com>

changelog_unreleased/misc/15709.md

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+#### Fix `cursorOffset` feature sometimes being catastrophically slow (#15709 by @ExplodingCabbage)
+
+Previously, Prettier's `cursorOffset` feature would be spectacularly slow in certain unfortunate circumstances (namely when the user's cursor was not contained within a leaf node of the AST, and the non-leaf node containing it was very large and being significantly reformatted by Prettier). As a consequence, if you used Prettier via an editor integration that used `cursorOffset` under the hood, your editor would sometimes inexplicably hang when you tried to format a file.
+
+All examples of this phenomenon that we are aware of should now be fixed, but bug reports of any further pathological examples would be welcome.