src: revert removal of SecureContext `_external` getter
This _external
getter is essential for some libs to work:
uWebSockets as an example.
It reverts this PR: https://github.com/nodejs/node/pull/20237 Use case which that PR broke: https://github.com/uNetworking/uWebSockets-bindings/blob/master/nodejs/src/uws.js#L508
Checklist
-
make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
commit message follows commit guidelines