`CanLoad` data provider

[robertbastian]

[sffc]

In Postcard, all non-experimental keys:

$ cargo run --release -p icu4x-datagen -- --markers all --locales full --format blob2 --deduplication maximal -o /tmp/icu4x_maximal.postcard
$ cargo run --release -p icu4x-datagen -- --markers all --locales full --format blob2 --deduplication retain-base-languages -o /tmp/icu4x_retain.postcard
$ du /tmp/icu4x_*.postcard 
19092   /tmp/icu4x_maximal.postcard
19120   /tmp/icu4x_retain.postcard

Plurals with 'plurals/ordinal@1' 'plurals/cardinal@1': size changes from 7157 to 7491