refactor: move operations on String.Pos.Raw to the String.Pos.Raw namespace

[TwoFX]

This PR moves many operations involving String.Pos.Raw to a the String.Pos.Raw namespace with the eventual aim of freeing up the String namespace to contain operations using String.ValidPos (to be renamed to String.Pos) instead.

This PR adds the String.ValidPos.set and String.ValidPos.modify functions.

After this PR, String.pos_lt_eq is no longer a simp lemma. Add String.Pos.Raw.lt_iff as a simp lemma if your proofs break.

[leanprover-community-bot]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase dca8d6d188f97b6bb0ec13e329a75a7bd0185684 --onto 705dac4f77aa60d749ee2b37d102518d772d6e53. You can force Mathlib CI using the force-mathlib-ci label. (2025-10-10 14:16:56)
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• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 7a47bfa2084eec2ec6dbc292d703e6297c2ccee8 --onto 14ff08db6f651775ead432d367b6b083878bb0f9. You can force Mathlib CI using the force-mathlib-ci label. (2025-10-14 19:41:37)
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[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 dca8d6d188f97b6bb0ec13e329a75a7bd0185684 --onto d3dda9f6d4428a906c096067ecb75e432afc4615. You can force reference manual CI using the force-manual-ci label. (2025-10-10 14:16:58)
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• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 7a47bfa2084eec2ec6dbc292d703e6297c2ccee8 --onto d3dda9f6d4428a906c096067ecb75e432afc4615. You can force reference manual CI using the force-manual-ci label. (2025-10-14 19:41:38)
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