feat: add whitespaceFilter for slotted and children node observation

[AKnassa]

What this does

Adds a whitespaceFilter you can pass to slotted and children, so purely-whitespace text nodes are left out of the results.

Why

Formatting your HTML across multiple lines creates invisible whitespace text nodes. Those were showing up in slotted and children results, so code had to filter them out by hand every time.

What changed

• New exported whitespaceFilter
• Works with both slotted and children node observation
• Opt-in — nothing changes unless you pass it

How to see it

Run the fast-element Playwright suite. 20 tests cover the children and slotted behaviour.

Closes #7139

[azure-pipelines]

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[azure-pipelines]

Azure Pipelines:
There may be pipelines that require an authorized user to comment /azp run to run.