Add a rule about cache definition language string being required for any new cache

[aspark21]

This is an easy one to miss and hopefully simple to add.

This throws a warning in our full build tests but it would be preferable if it could be caught earlier by moodle-plugin-ci / moodle-cs before code gets merged

e.g.

Given I navigate to "Plugins > Caching > Configuration" in site administration: debugging() message/s found:
String does not exist. Please check your string definition for cachedef_mappingmabinfo/local_sitsgradepush

[marinaglancy]

There are quite a few places where moodle expects a string with a magic name to be present:

• capabilities definitions in db/access.php
• caches definitions in db/caches.php
• message provider name in db/messages.php
• tag collection name in db/tag.php
• mnet publishers in db/mnet.php
• strings used in the addons in db/mobile.php
• names for the subplugin types if db/subplugins.json is present

As well as a lot of other places where string identifiers are returned/passed as parameters to the functions other than get_string - search areas, grade items, privacy providers, exceptions, and so on.

There has been a lot of work to identify all these places in https://mdlcode.dev extension for VSCode and it already raises "missing string" problems in all of these places.