[C# 12] New feature - primary constructors

[BillWagner]

Visual Studio release: 17.6p2

• Speclet
• PR

Task list:

• Add to What's new in C# 12
• Update language reference
  • Constructors page
  • records page
  • Class / Struct page
• update compiler messages
• Update guidance on class vs. struct vs. record vs. tuple
• consider tutorial on using primary constructors:
  • Include parameter maps to public property
  • Include parameter maps to private field
  • Include parameter used in one or more methods
• Consider pages on types and construction in fundamentals could be updated.

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Associated WorkItem - 69997

[BillWagner]

Note that dotnet/csharplang#7047 might be part of C# 12 as well.
That would impact primary constructors on record, record struct as well.