[lua-port] Update steam-easygrid to v4.0.0: Lua rewrite

[luthor112]

Easy SteamGrid

• Rewrite backend to use Lua
• Rewrite config management to use Millennium Library Manager

Task Checklist

Developer

• I am the original author or an authorized maintainer of this plugin.
• I have complied with all license requirements for the libraries used, including providing appropriate notices where necessary.
• My plugin is fully open source and does not depend on any external paid services, except for widely trusted and well-known platforms. Additionally, neither I nor anyone associated with me profits from any such services.

Plugin Functionality

• I have tested the plugin on both the Stable and Beta Steam update channels.

Backend Configuration

• No: I use a standard Millennium python backend in my plugin.
• Yes: I use a standard Millennium lua backend in my plugin.
• No: I use custom binaries that or rely on other FOSS projects that aren't written directly using Millennium's python backend.

Community Contribution

• I have tested and left feedback on two other plugin pull requests.
• I have added links to those feedback comments in this PR.

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Testing Instructions

• Verified by a third party on Steam Client Stable.
• Verified by a third party on Steam Client Beta.

[shdwmtr]

Thanks for the PR! I'll merge once Millennium is ready!