refactor: rename tauri::dev to is_dev and mark as constant fn

[Rajaniraiyn]

this enables the tauri::dev() can be used as const or static

Example

// Prevents additional console window on Windows in release, DO NOT REMOVE!!
#![cfg_attr(not(debug_assertions), windows_subsystem = "windows")]

use tauri::Manager;
use tauri_plugin_autostart::MacosLauncher;

static IS_DEV: bool = tauri::dev();

fn main() {
    tauri::Builder::default()
        .plugin(tauri_plugin_autostart::init(
            MacosLauncher::LaunchAgent,
            Some(vec![]),
        ))
        .plugin(tauri_plugin_shell::init())
        .setup(|app| {
            if IS_DEV {
                let window = app.get_webview_window("main").unwrap();
                window.open_devtools();
            } else {
                use tauri_plugin_autostart::ManagerExt;
                let autostart_manager = app.autolaunch();
                if !autostart_manager.is_enabled().unwrap() {
                    let _ = autostart_manager.enable();
                }
            }
            Ok(())
        })
        .run(tauri::generate_context!())
        .expect("error while running tauri application");
}

[amrbashir]

Thank you