cook doctor does not report non-numeric timer values

If I have a `.cook` file with a non-numeric timer like:

```
Cook for ~{a few%minutes}.
```

then `cook doctor` will not report any problems when validating the recipe, but [the playground](https://cooklang.github.io/cooklang-rs/) reports an error:

```
Error: Timer value is text: a few
╭─[playground]
│
1 │ Cook for ~{a few%minutes}.
┆ ──┬──
┆ │
┆ ╰───────────── expected a number here
──╯
```