Themes: Normalize Windows backslashes in theme directory paths.

[ArkaPrabhaChowdhury]

Use wp_normalize_path() in register_theme_directory() and search_theme_directories() so that paths are always stored and compared with forward slashes. This prevents a White Screen when a database dump created on Windows is loaded on a Unix system, where the stored backslash paths no longer match WP_CONTENT_DIR.

Fixes #29051

Trac ticket: https://core.trac.wordpress.org/ticket/29051

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