Impl Debug for AccessibilityNode.
Co-authored-by: delan azabani <dazabani@igalia.com>
Signed-off-by: Alice Boxhall <alice@igalia.com>

Example:
```
┌ Accessibility Tree
│  ├─ #0: GenericContainer (html_tag: "html")
│  │         children: [servo#1, servo#2]
│  │  ├─ servo#1: GenericContainer (html_tag: "head")
│  │  ├─ servo#2: RootWebArea (html_tag: "body")
│  │  │         children: [servo#3, servo#8]
│  │  │  ├─ servo#3: GenericContainer (html_tag: "div")
│  │  │  │         children: [servo#4]
│  │  │  │  ├─ servo#4: Heading (html_tag: "h1")
│  │  │  │  │         label: "This is an h1"
│  │  │  │  │         children: [servo#5]
│  │  │  │  │  └─ servo#5: TextRun
│  │  │  ├─ servo#8: GenericContainer (html_tag: "div")
│  │  │  │         children: [servo#6]
│  │  │  │  ├─ servo#6: Heading (html_tag: "h2")
│  │  │  │  │         label: "This is an h2"
│  │  │  │  │         children: [servo#7]
│  │  │  │  │  └─ servo#7: TextRun
```
Signed-off-by: Alice Boxhall <alice@igalia.com>