Add explicitSign parameter to all NumberFormats

[Albert221]

Is your feature request related to a problem? Please describe.
Let's say I'm building an app that shows a number difference, percentage or just a number. If the number is positive, I want to have the plus sign.

Describe the solution you'd like
NumberFormat.decimalPattern, .decimalPatternDigits, .decimalPercentPattern, percentPattern, simpleCurrency, currency, compactCurrency, the default constructor and maybe the scientificPattern (here I'm not sure) shall all have explicitSign constructor parameter. If they can display a minus sign for negative numbers, I don't see why only compact formats should be able to also show a plus sign for non-negative numbers. And this should be a constructor parameter and not only a mutable property on the NumberFormat, because I want to use this not only in code but also in flutter gen-l10n with ARB format and optionalParameters where I could set this flag to true.

Describe alternatives you've considered
Keeping two translation keys, one with a hardcoded plus sign, or passing a string to the translation, with a manually added plus sign.

Additional context
https://pub.dev/documentation/intl/latest/intl/NumberFormat-class.html

[mosuem]

Implementing this in package:intl is probably possible, happy to accept a PR ;)

For this to work with the ARB format, this should be included in the ARB spec, so that any ARB users could profit. The spec is currently hosted at https://github.com/google/arb-editor/blob/main/schemas/arb.json.