Add strict template variable support with global and per-variable configuration

[Kiran]

This proposal introduces optional strict handling for template variables.

Currently, Django silently renders missing variables as empty strings.
This feature would allow enabling a strict mode where undefined variables
raise an exception instead.

The proposal includes:

• A global strict template variable setting.
• Optional per-variable override behavior.
• Backward compatibility by keeping strict mode disabled by default.

Feedback is welcome before proceeding further.