feat: make Theorem an Inhabited instance

[wkrozowski]

This PR adds a default Inhabited instance to Theorem type.

The need to do so came up in #12296 , as Theorem is one of the entries of the structure which is the key entry of SimpleScopedEnvExtension.

[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 96ef09186f1cfe31bf5af69adb6ead4fad218ced --onto 5ec3b8c9d2fed98e6d78782664dd545785e868d4. You can force Mathlib CI using the force-mathlib-ci label. (2026-02-05 12:57:04)

[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 96ef09186f1cfe31bf5af69adb6ead4fad218ced --onto 42a0e9245300da6b3d971fa0033d726bb8e11cc3. You can force reference manual CI using the force-manual-ci label. (2026-02-05 12:57:05)