TimeBox a Property test

I'm building a `TimeboxedPropertyAttribute` for Hedgehog.Xunit. Its documentation:

> Runs tests until the time box is reached or the `tests` parameter is hit (100 by default). If no `size` is provided, a random `size` will be used for each test run.

Raison d'être: I have some tests that take a while to run. During development, I don't care _that_ much about correctness and will therefore timebox said tests. I'll flip them back to normal `property` tests before committing/merging. Since a test might only run once if the timebox is extremely short, I want a random size. (Always testing with `Size=0` isn't great.) This is pretty niche to my workflow, but LMK if any of this sounds interesting and anyone wants it merged upstream.

---

I'm currently implementing it via a hack (but without reflection):

```f#
...
let reportWith config =
Property.forAll invoke
>> match recheck with
| Some (size, seed) -> Property.reportRecheckWith size seed config
| None -> Property.reportWith config
match timeBox with
| Some timeBox ->
let config =
PropertyConfig.defaultConfig
|> PropertyConfig.withTests 1
|> withShrinks shrinks
let tests = tests |> Option.defaultValue 100
let rng = Random()
let report () =
101
|> rng.Next
|> Gen.resize
|> gens
|> reportWith config
let stop = DateTime.UtcNow + timeBox
let rec loop i r =
if DateTime.UtcNow >= stop || i >= tests || isNotOk r then
{ r with Tests = i }
else
report() |> loop (i + 1)
report() |> loop 1
...
```

Basically, I run the test once, see if the timebox is exceeded, and if not run it again, while manually incrementing the test count. A better solution would be to implement it in Hedgehog proper, but this hacky solution suited my needs. I'm willing to add it to Hedgehog if there's interest.

From [this conversation](https://github.com/hedgehogqa/fsharp-hedgehog/discussions/340#discussioncomment-1297254):

> Are you thinking this could be implemented with another combinator?

Yep!

> If so, would this be a property level thing? Or could this be supported with a property config option?

Right now I'm leaning in the direction of something on the level of property, i.e. a sibling of `checkWith`. Some of the qualities of a "TimeBoxedProperty" are at odds with the semantics and types of a normal `checkWith` test, so I don't think I can use the current `PropertyConfig` - I'll need a `TimeBoxedPropertyConfig`. For example, I can imagine a scenario where someone would want to run a test without specifying the testcount and only specifying the timebox. The result will be a test that runs as many times as it can for, say, 1 second. Currently `testcount` defaults to 100. Therefore it isn't enough to simply add an optional `timebox` field to `PropertyConfig`. `Size` also needs to be optional, and when its in the `None` case we'll generate random sizes.

Please feel free to consider this idea outside the scope of Hedgehog. I'm pretty sure Haskell doesn't do this!