Report printf formatter strings or specifiers

[rojepp]

Context: fsprojects/zarchive-fsharpbinding#603

I'm implementing highlighting for printf specifiers, and the feature could get greater coverage if symbols were reported from FCS. Don mentioned that FCS could be adjusted to report symbols for printf specifiers and/or whole formatting strings, so I'm adding this request here.

An implementation should perhaps already leave room for the new printf format, should it be accepted. We might like to do different highlighting depending depending on which type of specifier it is.

This new symbol would be returned in GetAllUsesOfAllSymbolsInFile, perhaps others.

[7sharp9]

I would like to understand where the symbol would be?

     §1  §2 §3
printfn "%s %i" "hello" 42

Would §1 §2 §3 all return symbols?

[dsyme]

I think the idea is that "format string resolutions" would be reported via a new method.

[dsyme]

I did a pair programming session with @dungpa and we made a rough prototype of the changes that would be needed to implement this: https://github.com/dsyme/FSharp.Compiler.Service/tree/printf-specifiers. @dungpa will take it from here :)