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This Pull Request integrates an `ICU4X` data provider API in our codebase, to make use of the internationalization APIs that this crate provides.
It changes the following:
- Creates an API for pluggable icu data providers at `Context` creation, adding an `Icu` struct to store the provider (and some other internationalization tools) at runtime.
- Slightly changes locale related functions to preserve the `Locale` type and ensure correctness. (Will make some other changes related to this).
- Integrates the `sys_locale` crate to fetch the current default locale of an user instead of always returning `en-US`.