feat: add proton-prefix-browser

[EvilNick2]

Proton Prefix Browser

I'm always digging into a game's Proton prefix to grab a save or tweak a config, and Steam only gives you Browse local files for the install folder. So I added a Browse Proton Prefix option right under it that opens the prefix in your file manager, plus a small button next to the gear on a game's page. It only shows up for games that actually have a prefix.

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.
• My plugin is unique, or provides additional or alternative functionality to plugins already on the store.

Backend Configuration

• No: I use a standard Millennium python 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.

Note: the backend is Lua (Millennium's Lua backend, backendType: "lua"). There's no Python, no custom binaries, and no external FOSS projects.

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.

---

Testing Instructions

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