Add TargetBuilder computation expression

It seems to support all kinds of expressions:
```fsharp
Target "All features" {
    let f x = x * x
    printfn "f 2 = %d" (f 2)

    try
        for i in [1..10] do
            try
                printfn "%d" i
                failwith "an exn"
            with e -> printfn "exn catched %A" e
    finally
        printfn "from finally"
}
```