Add child_count getter to CSSNode for efficient child counting (#197)

## Summary
Added a new `child_count` getter to the `CSSNode` class that efficiently
counts children without allocating an intermediate array, along with
comprehensive test coverage.

## Key Changes
- **New `child_count` getter**: Implements an efficient child counting
mechanism by iterating through siblings using the arena's internal
methods rather than materializing the full children array
- **Test coverage**: Added three test cases covering:
  - Counting children in a block declaration (3 properties)
  - Returning 0 for nodes with no children
  - Verifying consistency with the existing `children.length` property

## Implementation Details
The `child_count` getter uses a simple loop that traverses the sibling
chain starting from the first child, incrementing a counter until
reaching the end (index 0). This approach avoids the memory overhead of
creating an intermediate array when only the count is needed, making it
more efficient for performance-sensitive code paths.

https://claude.ai/code/session_01JZ7KyHoubj42U8DN2GRv7n

Co-authored-by: Claude <noreply@anthropic.com>

Author: bartveneman

src/api.test.ts

@@ -74,6 +74,26 @@ describe('CSSNode', () => {
 		})
 	})
 
+	describe('child_count', () => {
+		test('should return the number of children', () => {
+			const source = 'body { color: red; margin: 0; padding: 10px; }'
+			const root = parse(source, { parse_selectors: false, parse_values: false })
+			const block = root.first_child!.block!
+			expect(block.child_count).toBe(3)
+		})
+
+		test('should return 0 for a node with no children', () => {
+			const root = parse('', { parse_selectors: false, parse_values: false })
+			expect(root.child_count).toBe(0)
+		})
+
+		test('should match children.length', () => {
+			const source = 'body { color: red; } div { margin: 0; }'
+			const root = parse(source, { parse_selectors: false, parse_values: false })
+			expect(root.child_count).toBe(root.children.length)
+		})
+	})
+
 	describe('has_prelude', () => {
 		test('should return true for @media with prelude', () => {
 			const source = '@media (min-width: 768px) { body { color: red; } }'

src/css-node.ts

@@ -584,6 +584,17 @@ export class CSSNode {
 		return this.arena.has_children(this.index)
 	}
 
+	/** Count children without allocating an intermediate array */
+	get child_count(): number {
+		let count = 0
+		let child_index = this.arena.get_first_child(this.index)
+		while (child_index !== 0) {
+			count++
+			child_index = this.arena.get_next_sibling(child_index)
+		}
+		return count
+	}
+
 	/** Get all children as an array */
 	get children(): CSSNode[] {
 		let result: CSSNode[] = []