Open user's default browser for authentication

[Scarvy]

Instead of opening a new BrowserWindow to authenticate a user with Readwise, we would rather open the authentication URL in the user's default browser since most users will already be logged in.

Example of open in a user's own browser

[Scarvy]

Switched to using shell.openExternal for opening auth URL in user's default browser and any external links (ex. documentation or emailing support)

// Open external links in the default browser
mainWindow.webContents.setWindowOpenHandler(({ url }) => {
    shell.openExternal(url)
    return { action: 'deny' }
});