[css-highlight-api-1] Update examples

[ffiori]

[css-highlight-api-1] Update examples

Resolves issue #6331:

• Add 'div' variable definition in Example 2.
• Indent Example 1 and 2 like the others (one tab for contents of <style> and <script>).

Resolves issue #6333:

• Define Highlights h1 and h2 in Example 4, update the explanation accordingly.
• Change CSS.highlights.add() into CSS.highlights.set() in Example 4.
• Change style rules from p::highlight into ::highlight.

[ffiori]

@sanketj