Add Jinja2-style macros

[navrocky]

This PR adds support for Jinja2-style macros — reusable, parameterised template fragments — bringing inja closer to feature-parity with Jinja2 for common templating use cases.

{% macro input(name, value="", type="text") %}
  <input type="{{ type }}" name="{{ name }}" value="{{ value }}"/>
{% endmacro %}

{{ input("login") }}
{{ input("age", "18", "number") }}

Features

• Definition / call: {% macro name(args) %}...{% endmacro %} and {{ name(args) }}.
• Default parameter values: {% macro greet(name, greeting="Hello") %}...{% endmacro %}.
• Isolated scope: a macro body sees only its own parameters and the global input data — set variables and loop variables from the caller are not visible (matches Jinja2 semantics). Parameters never leak back
  to the caller.
• Macros calling macros, including recursion ({% macro foo(n) %}{% if n > 0 %}{{ foo(n - 1) }}{% endif %}{% endmacro %}).
• Hoisting from {% include %}: macros defined in an included template become callable in the including template after the {% include %} line. This works both for file-system includes and templates registered
  via env.include_template(...).
• Error handling: missing required argument → inja::RenderError; duplicate macro name / unmatched {% macro %} → inja::ParserError.

Closes #238