[17.2] Raw string literals

[BillWagner]

Now, you can surround string literals with an arbitrary number of " characters, matching opening and closing. That enables scenarios where the enbedded string needs quotes. See dotnet/csharplang#4304 and the feature spec

[BillWagner]

Notes where docs need changes:

• Built in reference types talk briefly about string literals.
• Programming with strings is a good location for the majority of the updates.
• Show the new syntax for this example
• Update the "what's new" article.