Fix #65431: Remove emoji from slugs during URL generation

[manishdhorepatil-art]

PROBLEM:
When creating posts with emoji in titles (like '🚀 Rocket'),
the slug becomes URL-encoded emoji characters causing broken URLs.

SOLUTION:
Strip emoji characters from slugs before sanitization.

CHANGES:

• Add _remove_emoji_from_slug_filter() function
• Hook at priority 9 (before main sanitization)
• Add remove_emoji_from_slug filter for backward compatibility
• Add 5 unit tests

HOW TO TEST:

1. Create new post with title: 'Test Emoji 🎉'
2. Check the generated slug
3. Slug should be 'test-emoji' (emoji removed)
4. Emoji in slug should NOT be URL-encoded

Trac Ticket - https://core.trac.wordpress.org/ticket/65431

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