refactor: rename ForwardPattern.dropPrefix? to ForwardPattern.skipPrefix?

[TwoFX]

This PR renames the function ForwardPattern.dropPrefix? to ForwardPattern.skipPrefix?

This function (s : String.Slice) -> Option s.Pos is not to be confused with String.Slice.dropPrefix? : (s : String.Slice) -> Option String.Slice.

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 8f6ade06ea2964d55625a9809f3349a0112aee04 --onto 61a3443a9569d548a235f785b9a33984bb7ff622. You can force Mathlib CI using the force-mathlib-ci label. (2026-03-19 13:45:29)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 8f6ade06ea2964d55625a9809f3349a0112aee04 --onto cfa8c5a036d6990635c6ec50b02d0e806995cec3. You can force reference manual CI using the force-manual-ci label. (2026-03-19 13:45:31)