timeout: Duration parsing does not support all float formats (e.g. hex)

GNU coreutils supports things like hex formatting, and arbitrary case for infinity parsing:
```
timeout 0x1.1 cat
timeout 0x1.1p-1 cat
timeout -k 0x10.1 1 cat
timeout INF cat
timeout InF cat
```

Not a huge deal, probably a pretty rare edge case that I doubt anybody cares about, but it seems worth it to use generic float parsing functions in `uucore/format` after they are fixed.