fix: add a default icon to notifications on linux

[betamos]

Currently, the notification api allows an optional icon. This has no effect on Windows and MacOS, where the icon is always taken from the application. This change has no effect on those platforms, but sets a default icon based on the binary path, which ensures notifications, by default, uses the app's icon.

This icon is also shown in dev mode if the app is installed. Otherwise, it just displays no icon.

What kind of change does this PR introduce?

• Bugfix
• Feature
• Docs
• New Binding issue #___
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change?

• Yes, and the changes were approved in issue #___
• No

Checklist

• When resolving issues, they are referenced in the PR's title (e.g fix: remove a typo, closes #___, #___)
• A change file is added if any packages will require a version bump due to this PR per the instructions in the readme.
• I have added a convincing reason for adding this feature, if necessary

Other information

[lucasfernog]

Please add a change file, check out the .changes folder to see how to do it. Basically:

---
"tauri": patch
---

Set notification icon to app icon on Linux.