Allow Interpolated strings to be `const` declarations

[BillWagner]

Constant interpolated strings

If all the placeholders are constants, an interpolated string can be a constant as well.

• Update interpolated strings tutorials for constant declarations
• Update const language reference
• update programming guide discussion of constants

[BillWagner]

In addition to the items noted above, the following use cases should be shown:

• Where the Obsolete attribute is explained, show the nameof expression as a way to recommend the updated API
• Consider an example for the DebuggerDisplay attribute and nameof