Add support for injecting dev toolbar apps

[BryceRussell]

ATP could provide a folder for authors to develop toolbar apps. For example:

package/
├── src/
│   └── toolbar/
│       ├── tool-one.js
│       ├── tool-two.js
│       └── tool-three.js
└── index.ts

defineTheme({
	// configure location of toolbar file/s
	toolbarDir: 'toolbar', (default)
})

Relevant Links:

• https://docs.astro.build/en/reference/dev-toolbar-app-reference/
• https://docs.astro.build/en/reference/integrations-reference/#adddevtoolbarapp-option