Fixed #36410 -- Added support for Template Partials to the Django Template Language.

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>

tests/template_tests/templates/partial_base.html

@@ -0,0 +1,2 @@
+{% block main %}
+{% endblock main %}

tests/template_tests/templates/partial_child.html

@@ -0,0 +1,9 @@
+{% extends 'partial_base.html' %}
+
+{% partialdef extra-content %}
+Extra Content
+{% endpartialdef %}
+
+{% block main %}
+Main Content
+{% endblock %}

tests/template_tests/templates/partial_examples.html

@@ -0,0 +1,15 @@
+{% partialdef test-partial %}
+TEST-PARTIAL-CONTENT
+{% endpartialdef %}
+
+{% block main %}
+BEGINNING
+{% partial test-partial %}
+MIDDLE
+{% partial test-partial %}
+END
+{% endblock main %}
+
+{% partialdef inline-partial inline %}
+INLINE-CONTENT
+{% endpartialdef %}