[feat][notification] Add dev mode option

[WSH032]

I know that the notification plugin can only work properly after installing the application via .msi or nsis. Otherwise, it can only use the identifier of other applications (such as powershell) to send notifications.

The problem here is that tauri determines whether to use dev mode by checking if the executable is in the target/ directory.

This works fine for typical Rust applications; however, when I compile it into a dynamic library for Python to call, this method fails because the executable is python in another directory.

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Here is my current solution: https://github.com/WSH032/pytauri/blob/953a44fd00b1b71e3585345d23e072225124488a/example/Tauri.windows.toml

But this is not ideal because tauri-cli will raise error for such invalid identifiers.

I hope the notification plugin can add a fn dev(self, enable: bool) option to allow users to manually set dev mode.

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I haven't checked the situation on Linux and macOS yet

[FabianLars]

A bit more context in #1502 (and the issues linked there)