function-keyword debug improvements

@vzarytovskii pointed out that FSharp core team cannot use `function` keyword:
```fsharp
let myFun =
function
| Some x -> x
| None -> "nothing"
```
...but instead always write explicit match:
```fsharp
let myFun = fun itm ->
match itm with
| Some x -> x
| None -> "nothing"
```
...because easier debugging.

My suggestion would be following improvements:

- when you hover over "function"-keyword at debug-time, it should display the omitted parameter value as tooltip.
- you should be able to attach a debug-point to function keyword itself (if not working already).