Improve conflicting Lua feature errors

[oiahoon]

Summary

• report the Lua runtime features that are actually enabled when they conflict
• keep luajit52 concise instead of also listing its implied luajit feature
• preserve the existing single-feature and no-feature behavior

Verification

• cargo check -p mlua-sys --features lua51,lua54 --offline (expected failure names both features)
• cargo check -p mlua-sys --features lua55,lua53,luau --offline (expected failure names all three features)
• cargo check -p mlua-sys --features lua54,luajit52 --offline (expected failure names luajit52 once)
• cargo check -p mlua-sys --features lua54,vendored --offline
• cargo test --features lua54,vendored --offline
• cargo clippy -p mlua-sys --all-targets --features lua54,vendored --offline -- -D warnings
• rustfmt --edition 2024 --check mlua-sys/build/main.rs

Closes #729