Support # placeholder in ICU messages

### Which @angular/* package(s) are relevant/related to the feature request?

localize

### Description

The ICU message format specifies a placeholder for the "current number" that can be used in plural rules. Example:

```html

Updated {currentItem().timeInMinutes(), plural,
one {# minute ago}
other {# minutes ago}
}

```

But when used in Angular, this renders as `Updated # minute ago` with a literal `#` in the output. It looks like Angular currently requires repeating the number expression for each case, separated by spaces to not confuse the nested braces:

```html

Updated {currentItem().timeInMinutes(), plural,
one { {{currentItem().timeInMinutes()}} minute ago}
other { {{currentItem().timeInMinutes()}} minutes ago}
}

```

References:

* https://unicode-org.github.io/icu/userguide/format_parse/messages/#messageformat
* https://unicode-org.github.io/icu-docs/apidoc/dev/icu4c/classicu_1_1PluralFormat.html
* https://www.npmjs.com/package/@messageformat/parser

### Proposed solution

Support the standard `#` placeholder. This would be a breaking change.

### Alternatives considered

n/a, keep as is since it would be breaking.

Start in the @angular/localize package and trace how ICU plural messages are parsed and rendered. Compare the current behavior with the ICU references linked in the issue, then locate relevant existing tests or entry points. Done means # is interpreted as the current plural number in one and other cases without requiring repeated expressions, with the breaking change accounted for.

Written by the indexing model from the issue text.