#51543: Add is_sitemap()

[masteradhoc]

This patch introduces is_sitemap() as a conditional query tag and adds the $is_sitemap property to WP_Query, following the established pattern of is_robots() and is_favicon().

Trac ticket: https://core.trac.wordpress.org/ticket/51543

Use of AI Tools

AI assistance: Yes
Tool(s): GitHub Copilot
Model(s): Claude Sonnet 4.6
Used for: Initial implementation scaffolding. All code was reviewed, adjusted, and tested by me.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

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[github-actions]

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Props masteradhoc, mukesh27.

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[masteradhoc]

@mukeshpanchal27 to make reviewing easier i split the second trac ticket to a own PR, see: #12150