[wasm][icu] Allow basic ICU sharding in WASM app like Blazor does it + custom files

[ilonatommy]

Blazor loads ICU datafile based on user's config and if it's empty: based on the Browser's locale. WASM is always loading full ICU.
We can copy Blazor's behavior by letting users set the shard in MsBuild + allow them to use custom ICU build. Moreover, we need to add a how-to for ICU.dat building, something like here: #53239 (comment)

[dotnet-issue-labeler]

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[ghost]

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Issue Details

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Blazor loads ICU datafile based on user's config and if it's empty: based on the Browser's locale. WASM is always loading full ICU.
We can copy Blazor's behavior by letting users set the shard in MsBuild + allow them to use custom ICU build. Moreover, we need to add a how-to for ICU.dat building, something like here: #53239 (comment)

Author:	ilonatommy
Assignees:	ilonatommy
Labels:	arch-wasm, area-System.Globalization
Milestone:	8.0.0

[ilonatommy]

Solved by #80421.