F#: Allow triple-quoted strings to interpolate string literals (.NET 5 feature)

[rdipardo]

Since the release version 5.0, F# handles interpolated strings with as much facility as C# has done for years.

Lexilla's F# lexer doesn't yet recognize all the potential use cases of this new language feature.

In particular, a triple-quoted string may be prefixed with the $ interpolation sigil; this is the only way to interpolate a value that is itself a string literal ℹ️ :

> $"""Date: {System.DateTime.Now.ToString("yyyy-MM-dd")}""" ;;
val it : string = "Date: 2021-07-28"

The internal string should not interrupt the surrounding style region; but that's currently not the case:

This patch set implements and tests interpolated string literals in the above context:

• LexFSharp-EmbedInterpolatedStrLits.zip

---

ℹ️

> $"Date: {System.DateTime.Now.ToString("yyyy-MM-dd")}" ;;

  $"Date: {System.DateTime.Now.ToString("yyyy-MM-dd")}" ;;
  --------------------------------------^

stdin(2,39): error FS3373: Invalid interpolated string. Single quote or verbatim string literals may not be used in interpolated expressions in single quote or verbatim strings. Consider using an explicit 'let' binding for the interpolation expression or use a triple quote string as the outer string literal.