feat: add String simprocs to cbv

[wkrozowski]

This PR adds String-specific simprocs to cbv tactic.

[wkrozowski]

!bench

[leanprover-radar]

Benchmark results for 4d4b0cc against a32be44 are in. There are no significant changes. @wkrozowski

• 🟥 build//instructions: +1.3G (+0.01%)

Medium changes (1🟥)

• 🟥 compiled/workspaceSymbolsNewRanges//instructions: +16.1M (+2.17%)

Small changes (1🟥)

• 🟥 build/module/Lean.Meta.Tactic.Cbv.Util//instructions: +91.4M (+7.34%) (reduced significance based on *//lines)

[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 a32be44f909683a2f5dff4bb955dfa986bbc9b1c --onto e8048291010c815d0d30926924e7ad7afc18b1c0. You can force Mathlib CI using the force-mathlib-ci label. (2026-03-11 17:04:41)

[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 a32be44f909683a2f5dff4bb955dfa986bbc9b1c --onto ff6816a854ef5cf2966519452d03933785f2fdeb. You can force reference manual CI using the force-manual-ci label. (2026-03-11 17:04:43)