feat: use a computableExt instead of a noncomputableExt

[Rob23oba]

This PR removes noncomputableExt and replaces it with a computableExt which records successfully compiled declarations. Previously, you had to be careful to addNoncomputable after addDecl if the declaration is not compiled but now, there is no such responsibility anymore.

[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 58408f99cb2b6cdd72be94b5fbc26881e49502dc --onto 110db9cb751afaee8b2ac344887d6c7e632f77b4. You can force Mathlib CI using the force-mathlib-ci label. (2026-08-04 14:33:12)

[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 58408f99cb2b6cdd72be94b5fbc26881e49502dc --onto 23393b959b33e3a8d15796b2397f8a04c315b9f4. You can force reference manual CI using the force-manual-ci label. (2026-08-04 14:33:13)