Add support for the "parsercache" section flavor
Previously, parsercache sections were given the "external" flavor, which was a sensible default choice given that these sections are included in the externalLoads config property.
However, we now want to be able to assert safety invariants on / across parsercache sections, so we need a clear way to identify them. Imbuing them with their own flavor is a natural way to do that.
Note that parsercache sections are still included in the externalLoads config attribute, and there is no intention of changing this in the near term (i.e., this is transparent to mediawiki-config).
Bug: T383324