Introduced `{% partialdef %}` and `{% partial %}` template tags to
define and render reusable named fragments within a template file.
Partials can also be accessed using the `template_name#partial_name`
syntax via `get_template()`, `render()`, `{% include %}`, and other
template-loading tools.
Adjusted `get_template()` behavior to support partial resolution, with
appropriate error handling for invalid names and edge cases. Introduced
`PartialTemplate` to encapsulate partial rendering behavior.
Includes tests and internal refactors to support partial context
binding, exception reporting, and tag validation.
Co-authored-by: Carlton Gibson <carlton@noumenal.es>
Co-authored-by: Natalia <124304+nessita@users.noreply.github.com>
Co-authored-by: Nick Pope <nick@nickpope.me.uk>
