To prepare for the optimization of template-only changes, this commit
refactors the `ComponentDecoratorHandler`'s handling of template parsing.
Previously, templates were extracted from the raw decorator metadata and
parsed in a single operation.
To better handle incremental template updates, this commit splits this
operation into a "declaration" step where the template info is extracted
from the decorator metadata, and a "parsing" step where the declared
template is read and parsed. This allows for re-reading and re-parsing of
the declared template at a future point, using the same template declaration
extracted from the decorator.
PR Close#40561
52aeb5326d