src: move Environment::context out of strong properties

Rename `ENVIRONMENT_STRONG_PERSISTENT_PROPERTIES` to
`ENVIRONMENT_STRONG_PERSISTENT_TEMPLATES`, and move `context`
out of the list, so that the data can be iterated separately.

PR-URL: https://github.com/nodejs/node/pull/27430


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Refael Ackermann <refack@gmail.com>