Add Ayu theme

[shamsghi]

Summary

• add full Ayu theme extension as a new submodule at extensions/ayu-theme
• register ayu-theme in extensions.toml at version 0.1.0
• include Ayu Dark, Ayu Mirage, and Ayu Light from https://github.com/shamsghi/Ayu-in-Zed

Notes

• this theme bundles all of the ayu themes in one extension, unlike other existing ayu themes that are more like variants of the official ayu theme (https://github.com/k4yt3x/zed-theme-ayu-darker & https://github.com/jansol/zed-ayu-glass)

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[shamsghi]

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