Add t81 extension file to ZX81 system

[wwark]

Description

Add t81 extension file to ZX81 system
It is present on the doc https://docs.libretro.com/library/eightyone/#extensions

Fixes # (issue)

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested Locally?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Tested on RGB30 i can load t81 extension file on ZX81 core
• Test B

Test Configuration:

• Build OS name and version: Ubuntu 22.04
• Docker (Y/N): N
• JELOS Branch: Dev
• Any additional information that may be useful:

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation

Note: This PR template is adapted from embeddedartistry

[sydarn]

Thank you!