Added `.clang-tidy` config as discussed in
[RFC](https://discourse.llvm.org/t/rfc-create-hardened-clang-tidy-config-for-clang-tidy-directory/87247).
Added `bugprone`, `readability`, `modernize`, `performance` checks that
didn't create many warnings.
Fixed minor warnings to make `/clang-tidy` directory complaint with
`clang-tidy-20`.
Disabled checks will be enabled in future PRs after fixing their
warnings.