move tslib to dependencies

[nicklloyd]

Summary

• tsconfig.json sets importHelpers: true, so emitted code does require("tslib") at runtime (61 files in dist/ currently).
• tslib was only listed in devDependencies, so consumers installing @apideck/portman would fail with Cannot find module 'tslib'.
• Moves tslib into runtime dependencies and updates the lockfile.

Options considered

1. Keep as-is (now that tslib is a dep) — recommended, smallest change. (chosen)
2. Drop importHelpers + leave tslib out — simpler deps, larger dist/ (each file inlines its own helpers).
3. Bump target to es2019 (Node 12+ supports it, matches engines) — eliminates most helpers entirely, but it's a broader change that could surface other issues and is outside the scope of this fix.

Test plan

• npm run build succeeds
• Emitted dist/**/*.js files resolve tslib from runtime deps
• Install from a tarball in a clean consumer project and require the package

[ChrisBrenton]

@thim81 Any chance of this being looked at? This is causing failures for many people it seems.