Allow extensions to register additional namespaces in extension.json

Extensions can register additional namespaces by setting the
"TemplateStylesNamespaces" attribte in their extension.json. This change is
analogous to Ia5d34cb78fa6af.

There isn't really a simpler way to do this here, as the config setting uses
namespaces as keys, while the attribute is using them as values. Furthermore,
keys with falsey values are ignored in the config setting, and attributes can't
use the same setting architecture, as array_merge_recursive doesn't maintain
numeric keys.

Bug: T200914
Change-Id: I2fa9b822ee39bcc5f95a293c8c4aad4d53ede30a