As mentioned in https://github.com/boa-dev/boa/pull/2848#issuecomment-1518909512, this uses our new default ICU4X data to replace `char::is_start` and `char::is_continue` from the `boa_unicode` crate with the [`icu_properties`](https://crates.io/crates/icu_properties) crate.
Note that this doesn't deprecate `boa_unicode` yet, since that'll require some discussion about how to proceed with a now unused sub-crate.
We currently use `unicode_normalization` to handle the `String.prototype.normalize` method. However, the crate doesn't support UTF-16 as a first class string, so we had to do some hacks by converting the valid parts of a string to UTF-8, normalizing each one, encoding back to UTF-16 and concatenating everything with the unpaired surrogates within. All of this is obviously suboptimal for performance, which is why I leveraged the `icu_normalizer`, which does support UTF-16 input, to replace our current implementation.
Additionally, this allows users to override the default normalization data if the `intl` feature is enabled by providing the required data in the `BoaProvider` data provider.
José Julián Espina