Replace PyICU with Rust icu_segmenter crate

[anoadragon453]

Closes #18282.

Replaces the PyICU python wrapper around the C++ icu library with a native Rust solution; the icu_segmenter crate, which is part of the broader icu crate.

This eliminates the need for downstream users to install the libicu package if they want to have improved user search.

The model decided to use the default constructor for WordSegmenter, and I agree. Our "data" is the user's search query, which is always parsed on the fly and not stored. So it's OK if it changes between ICU releases.

Note: This PR was mostly written by OpenAI Codex, before being reviewed by myself. Commits written by Codex are marked as those made by @anoadragon453-codex. Please use extra caution when reviewing them.

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