Breaking: Add zero-cost TypeScript node type narrowing layer (#205)

## Summary

This PR introduces a comprehensive TypeScript type narrowing system for
CSS AST nodes, enabling type-safe node handling without runtime
overhead. The implementation provides both type predicate functions and
a discriminated union type for exhaustive narrowing patterns.

## Key Changes

- **New `src/node-types.ts`**: Defines 39 specialized node interfaces
that extend `CSSNode` with precise property types, removing spurious
`undefined` values from type-specific properties
- Each interface overrides property types to reflect what a specific
node type actually returns
- Examples: `DeclarationNode.property` is `string` (not `string |
undefined`), `DimensionNode.unit` is `string` (not `string | undefined`)

- **Type predicate functions**: 39 individual `is_*()` functions for
runtime type narrowing
  - Each compiles to a single integer comparison (zero heap allocation)
  - All are individually tree-shakeable named exports
- Enable idiomatic TypeScript patterns: `if (is_declaration(node)) {
node.property /* string */ }`

- **`AnyCssNode` discriminated union**: Enables exhaustive switch-based
narrowing without explicit type predicates
- Allows automatic type narrowing in switch statements based on
`node.type`
  - Useful for walk callbacks and visitor patterns

- **Comprehensive test suite** (`src/node-types.test.ts`): 
  - Runtime behavior tests for all major type predicates
- Compile-time type assertion tests using `expectTypeOf` to verify
narrowing works correctly
  - Tests for selector subtypes and complex nested structures

- **Updated exports** in `src/index.ts`: All node type interfaces and
predicate functions are now publicly exported

- **Updated `src/walk.ts`**: Walk callback parameter type changed from
`CSSNode` to `AnyCssNode` for better type inference in user code

- **Updated `src/constants.ts`**: Added missing `UNICODE_RANGE` export

## Implementation Details

- All interfaces are erased at compile time — zero bytes in the JS
output
- Type predicates compile to single integer comparisons with no runtime
overhead
- The design follows TypeScript best practices for discriminated unions
and type guards
- Comprehensive documentation in comments explains usage patterns and
benefits

https://claude.ai/code/session_01AutHjiFtdrfyBLJ64JwXnM

---------

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

Author: bartveneman

API.md

@@ -94,11 +94,6 @@ The `CommentInfo` object passed to `on_comment` contains:
 - `nth_a` - The 'a' coefficient from An+B expressions like `"2n"` from `:nth-child(2n+1)` (for NTH_SELECTOR)
 - `nth_b` - The 'b' coefficient from An+B expressions like `"+1"` from `:nth-child(2n+1)` (for NTH_SELECTOR)
 
-**Attribute Selector Properties:**
-
-- `attr_operator` - Attribute operator constant (for ATTRIBUTE_SELECTOR): `ATTR_OPERATOR_NONE`, `ATTR_OPERATOR_EQUAL`, etc.
-- `attr_flags` - Attribute flags constant (for ATTRIBUTE_SELECTOR): `ATTR_FLAG_NONE`, `ATTR_FLAG_CASE_INSENSITIVE`, `ATTR_FLAG_CASE_SENSITIVE`
-
 **Methods:**
 
 - `clone(options?)` - Clone node as a mutable plain object with children as arrays
@@ -902,48 +897,6 @@ const pseudoClass2 = ast2.first_child.first_child
 console.log(pseudoClass2.has_children) // false - no function syntax
 ```
 
-## Attribute Selector Constants
-
-### Attribute Selector Operators
-
-Use these constants with the `node.attr_operator` property to identify the operator in attribute selectors:
-
-- `ATTR_OPERATOR_NONE` (0) - No operator (e.g., `[disabled]`)
-- `ATTR_OPERATOR_EQUAL` (1) - Exact match (e.g., `[type="text"]`)
-- `ATTR_OPERATOR_TILDE_EQUAL` (2) - Whitespace-separated list contains (e.g., `[class~="active"]`)
-- `ATTR_OPERATOR_PIPE_EQUAL` (3) - Starts with or is followed by hyphen (e.g., `[lang|="en"]`)
-- `ATTR_OPERATOR_CARET_EQUAL` (4) - Starts with (e.g., `[href^="https"]`)
-- `ATTR_OPERATOR_DOLLAR_EQUAL` (5) - Ends with (e.g., `[href$=".pdf"]`)
-- `ATTR_OPERATOR_STAR_EQUAL` (6) - Contains substring (e.g., `[href*="example"]`)
-
-### Attribute Selector Flags
-
-Use these constants with the `node.attr_flags` property to identify case sensitivity flags in attribute selectors:
-
-- `ATTR_FLAG_NONE` (0) - No flag specified (default case sensitivity)
-- `ATTR_FLAG_CASE_INSENSITIVE` (1) - Case-insensitive matching (e.g., `[type="text" i]`)
-- `ATTR_FLAG_CASE_SENSITIVE` (2) - Case-sensitive matching (e.g., `[type="text" s]`)
-
-#### Example
-
-```javascript
-import {
-	parse_selector,
-	ATTRIBUTE_SELECTOR,
-	ATTR_OPERATOR_EQUAL,
-	ATTR_FLAG_CASE_INSENSITIVE,
-} from '@projectwallace/css-parser'
-
-const ast = parse_selector('[type="text" i]')
-
-for (let node of ast) {
-	if (node.type === ATTRIBUTE_SELECTOR) {
-		console.log(node.attr_operator === ATTR_OPERATOR_EQUAL) // true
-		console.log(node.attr_flags === ATTR_FLAG_CASE_INSENSITIVE) // true
-	}
-}
-```
-
 ---
 
 ## `@projectwallace/css-parser/string-utils`