Use ICU on Windows when available

[safern]

Contributes towards: #826
Fixes: #4673

Mono is still defaulted to use NLS because we don't produce a shim in the libraries Windows build. Once the shim is linked into the mono runtime I'll update the mono code to use ICU when available. (cc: @marek-safar)

FYI: @GrabYourPitchforks @ericstj

[ghost]

Tagging @tarekgh, @safern as an area owner

[GrabYourPitchforks]

FYI I recently changed some of these files in #34616. There will be conflicts until you merge in origin/master.

[mjsabby]

Another question. We use the sysglobl APIs to create additional cultures and reparent them to existing ones in most cases. This afaik ultimately are just additions to the registry. Will such a feature will be available to ICU?

[tarekgh]

@mjsabby

Another question. We use the sysglobl APIs to create additional cultures and reparent them to existing ones in most cases. This afaik ultimately are just additions to the registry. Will such a feature will be available to ICU?

Unfortunately, we'll be limited to what the underlying ICU library is providing to us. I checked with the Windows team and learned custom locales are not supported by ICU when running on Windows. It would be nice if you can tell why you are using such custom locales?
If it is really very important to you then you may use the ICU app local feature which will allow you to load the app local version of the ICU and not the system one. And you'll need to build and install your own ICU customized locales data file. Although this can be non-trivial it is not impossible to do.

CC @jefgen @ShawnSteele

[mjsabby]

@tarekgh For business and legal reasons we need to define few markets differently than Windows. So we definitely need to use the app local feature.

The question is how much more complicated is it that than the current process which is calling sysglobl APIs.

Is the app-local feature documentation available somewhere?

[tarekgh]

For business and legal reasons we need to define few markets differently than Windows.

Is this mean you need to update some locale data but not adding any more locales?

Is the app-local feature documentation available somewhere?

This is not implemented yet. we are going to implement it next couple of weeks. Basically app local feature is the app will carry its own copy of ICU. Using the tool https://github.com/unicode-org/icu/blob/master/docs/userguide/icu_data/buildtool.md I believe you can build your own customized copy of ICU data and use it. I didn't play with this tool before but I think it will give you full control over the locale data for your app. Also, you'll have control when you can pick a newer version of the ICU at the time you like.
Also, you can just clone the ICU repo and edit the locale data you need and then build it. this will not need to use the tool I think as you'll build a regular way.