Add custom link handlers

[Airyzz]

Description

Adds the ability for the Tauri backend to handle the opening of links. This allows for certain links to be opened in a specific application, as well as fixing an issue where clicking a link in Cinny Desktop would navigate to that URL inside the tauri window.

I added an example link handler, for steamcommunity.com, and store.steampowered.com, which opens these urls inside the Steam app, if Steam is running on the user's computer.

Type of change

• 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

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
• My changes generate no new warnings

Related: cinnyapp/cinny#790

[Airyzz]

Changes to the Cinny submodule can be discarded, the necessary changes are in the pull request over on the main repo

[kfiven]

As cinnyapp/cinny#790 is closed.