ICU standards for numbers formatting works partially

As described in the readme:

> Message format is strictly implemented by ICU standards.

But I tested the number formatting from here:

> https://unicode-org.github.io/icu/userguide/format_parse/numbers/skeletons.html

and most of the examples don't work, such as:

```
{value, number, ::.####}
```

or

```
.00/w
```

But

```
{value, number, ::.0####}
```

works

Start with the number-formatting implementation and the README statement that message format follows ICU standards, then reproduce the examples from the linked ICU number skeletons guide, including `::.####`, `.00/w`, and `::.0####`. Done means the supported examples behave according to ICU expectations or the README clearly documents the limitation.

Written by the indexing model from the issue text.