[6.x] More code splitting

[jasonvarga]

• Making NavBuilder and PageTree async components allows @he-tree/vue to be code-split.
• Making Modal an async component and moving the css allows vue-final-modal to be split.
• Making the TimeFieldtype an async component allows imask to be split.
• Using dynamic imports in the Toasts class allows @hoppscotch/vue-toasted to be split.
• Using dynamic imports in the Echo class allows laravel-echo and pusher to be split.

The $toast API methods are now technically async. But you would never really chain methods, and having a slight delay in the toast appearing doesn't hurt anything.

Before this, the js file was 2,266.68 kB │ gzip: 607.05 kB.
After these changes, the js file is 1,923.01 kB │ gzip: 509.28 kB.