Lazy-initialize DateTimeFormatInfoScanner.m_dateWords

[stephentoub]

All the checks were already in place to lazy-initialize, and only a subset of cultures have date words.

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[Copilot]

Pull request overview

This PR updates DateTimeFormatInfoScanner to lazily initialize its internal date word collection, avoiding unnecessary allocations for cultures that have no date words, and cleans up documentation and implementation details.

Changes:

• Remove the legacy banner comment and replace it with an XML <summary> comment describing DateTimeFormatInfoScanner and its role in DateTime.Parse.
• Change m_dateWords from an eagerly initialized List<string> to a nullable List<string>? that is lazily initialized where needed, using collection expressions (m_dateWords ??= []).
• Simplify GetDateWordsOfDTFI by using pattern matching on m_dateWords and List.ToArray() instead of manual array allocation and copying, and fix a spelling mistake in a comment (“Enumarate” → “Enumerate”).