npm install esbuild@0.18.20 in test/package/browser/template

Had to downgrade version for explicit dev dependency for `esbuild` to
match version used by `@playwright/experimental-ct-react` (0.18.20).
It was causing 'Error: Expected "0.19.5" but got "0.18.20"' errors
in CI, see job log [1].

[1] https://github.com/ably/ably-js/actions/runs/7984893618/job/21802628829