Closes #ISSUE

Release Notes:

- N/A

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I just make a little change to improve `index_for_position` to support
return closest index for position.

I need this method to measure for position cursor in multi-line mode
TextInput.

https://github.com/longbridge/gpui-component/pull/583


https://github.com/user-attachments/assets/c69d098e-d2cb-4053-b739-6c7dd666e769

Before this change, GPUI have `LineLayout::closest_index_for_x` method
for unwrapped line case.


https://github.com/zed-industries/zed/blob/d1be419fff415329b38f26aff90488700702c82a/crates/gpui/src/text_system/line_layout.rs#L58-L94

This change is equivalent to making `index_for_position` have a
corresponding method to get the closest index like `index_for_x`.