Add more string functions - StringBuilder extension

Description

https://github.com/fsprojects/FSharpPlus/issues/158#issuecomment-512607693

For the StringBuilder extension, it will be used like below with the String.build function from my String.fs:

let s =
  String.build (fun builder ->
    builder.printfn "%s" "hello"
    builder.printfn "%i%i" 4 2
    ...
  )

The motivation to use this extension is to avoid the repetitive use of ignore caused by the fact StringBuilder's Write and WriteLine returns the StringBuilder for method chaining. Also, sb.WriteLine(sprintf "%i" 42).WriteLine(sprintf "%s" "foo").ToString() looks odd and ugly imo, but this might be just a personal impression.

https://github.com/fsprojects/FSharpPlus/issues/158#issuecomment-512721780

the build and the StringBuilder functions looks nice to me, but let's open another issue for StringBuilder extensions as at the moment we don't have any of them, and it might require more discussion.

Referece implementation:

From cannorin/prelude.fs:

let build (builder: StringBuilder -> unit) =
  let sb = new StringBuilder()
  builder sb
  sb.ToString()

type StringBuilder with
  member inline this.printf format =
    Printf.kprintf (fun s -> this.Append s |> ignore) format

  member inline this.printfn format =
    Printf.kprintf (fun s -> this.AppendLine s |> ignore) format

Start with the linked discussion in fsprojects/FSharpPlus issue 158 and compare its proposed StringBuilder extension with the reference implementation in cannorin/prelude.fs. Check the existing String.fs and extension organization, then determine whether String.build plus printf and printfn are the agreed scope; done means the selected functions are available with matching behavior and coverage.