This pull request adds XIM (X Input Method) support to x11 platform.

The implementation utilizes [xim-rs](https://crates.io/crates/xim), a
XIM library written entirely in Rust, to provide asynchronous XIM
communication.
Preedit and candidate positioning are fully supported in the editor
interface, yet notably absent in the terminal environment.

This work is sponsored by [Rainlab Inc.](https://rainlab.co.jp/en/)

Release Notes:
- N/A

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Signed-off-by: npmania <np@mkv.li>