Keep track of verbatim string in Untyped Syntax Tree.

[nojaf]

Is your feature request related to a problem? Please describe.
Related to Fantomas bug fsprojects/fantomas#560.
In short, the AST does not keep track of the @ character before a string.

let s = @"\"

Creates

SynConst.String("\", tmp.fsx (1,8--1,12) IsSynthetic=false), tmp.fsx (1,8--1,12) IsSynthetic=false)

Describe the solution you'd like
Can we extend SynConst.String to indicate the leading @ was present?

Describe alternatives you've considered
In Fantomas, we currently detect the @ via the tokens.

Additional context
If approved, I'm willing to try and implement this myself.

[nojaf]

Hello @cartermp, I would be interested in an FCS release because of this. Do I need to raise this somewhere formally?

[cartermp]

We don't have a good process in place yet. We're closed down on the VS 16.9 release, which will correspond to FCS 39 - at least for now - but do have several ways to work with that. We (myself, @KevinRansom, @dsyme, etc.) need to establish what the policy for releases will be though. The past isn't a great guide since there could be releases a week apart or 6 months apart. But we do think that only following the schedule of the F# tools for VS isn't sufficient either.