[js_interop] Add an api.txt golden (#609)

Track `package:js_interop`'s public API surface with `package:api_summary`
(`js_interop/tool/api.txt`, excluded from the published archive via
`.pubignore`) so signature and member changes are visible in PR diffs,
matching `package:web` (#590).

CI runs `test/api_test.dart` on the VM against the `dev` SDK since
`js_interop` defaults to browser and Node test platforms and `api_summary`
output tracks SDK analyzer updates.