Fix nested element selector parsing in DeclarationList context

[Copilot]

The CSS parser was failing to correctly parse nested rules that begin with element selectors in DeclarationList context. Consider:

/* This didn't work - p was treated as a property, not a selector */
main {
   color: blue;
   p {
      margin-top: 0;
   }
}

The issue was in the DeclarationList.js parser which only recognized & (nesting selector) as the start of a nested rule. Any other selector like p, div:hover, .class, etc. would be incorrectly parsed as a malformed declaration and converted to Raw content.

Changes made:

1. Enhanced selector detection: Updated DeclarationList.js to use comprehensive isSelectorStart() logic instead of just checking for &

2. Improved lookahead logic: Added sophisticated token lookahead in isElementSelectorStart() to distinguish between:

   • Properties: p: value; or p: value
   • Element selectors: p { ... }
   • Pseudo-class selectors: div:hover { ... }

3. Fixed EOF handling: Properly handle end-of-input cases to ensure p:v (without semicolon) is correctly parsed as a declaration, not a selector

4. Updated test fixtures: Modified existing test cases to reflect the correct parsing behavior

Before:

// Parsing "color: blue; p { margin: 0; }" in DeclarationList context
{
  children: [
    { type: "Declaration", property: "color", ... },
    { type: "Raw", value: "p { margin: 0; }" }  // ❌ Raw content
  ]
}

After:

// Parsing "color: blue; p { margin: 0; }" in DeclarationList context  
{
  children: [
    { type: "Declaration", property: "color", ... },
    { type: "Rule", prelude: { /* p selector */ }, ... }  // ✅ Proper Rule
  ]
}

The fix now correctly handles all selector types in nested contexts while preserving proper distinction between properties and selectors.

Fixes #84.

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Hi @Copilot!, thanks for the Pull Request

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