Support multi-flavour mode with matrix generation

[sgoudham]

Currently, it seems like the flavors object is not present when running in multi-flavour mode.

This is a use-case initially raised by @tecandrew (the current catppuccin/zed maintainer) where we're trying to generate all the configuration for 4 flavours but split them out into 2 separate variants and generate 2 files instead of 8.

Reproducible Template

---
whiskers:
  version: 2.0.0
  matrix:
    - variant: ["italics", "no-italics"]
  filename: "themes/catppuccin-{{ variant }}.json"
---

{% for _, flavor in flavors -%}
{{flavor.name}} {% if variant == "no-italics" %} (no italics){% endif %}
{% endfor %}

Actual Output

└ $ whiskers cava.tera
Error: Multi-output render failed

Caused by:
    0: Main template render failed
    1: Failed to render 'cava.tera'
    2: Variable `flavors` not found in context while rendering 'cava.tera'

Expected Output

Directory Structure:

themes/
	catppuccin-italics.json (contains rendered template for all 4 flavours)
	catppuccin-no-italics.json (contains rendered template for all 4 flavours)

catppuccin-italics.json

Latte
Frappé
Macchiato
Mocha

catppuccin-no-italics.json

Latte (no-italics)
Frappé  (no-italics)
Macchiato  (no-italics)
Mocha  (no-italics)