Show scan duration in the terminal summary

Since `lockcheck` is an incredibly fast, zero-dependency tool, it would be a nice flex to show exactly how fast it runs in the final terminal output!
We should calculate the time elapsed from the start of the `scan()` function to the end, and print it in the terminal summary panel.

**Acceptance Criteria:**
-
- Use `performance.now()` or `Date.now()` around the execution in `bin/lockcheck.js` (or populate it in the `stats` object inside `index.js`).
- Update the `src/reporters/terminal.js` summary section to display something like: `⏱️ Scanned in 0.42s`.