Specify customWidth instead of extraWidth for elements like custom emoji

We have a use case where we are replacing emojis with custom pixel based emoji `img` elements, which have a set width, and it would be nice to have the `RichInlineItem`s have an option to flow these naturally as well.
My current workaround is to use a placeholder symbol like □ and subtract its width manually through `extraWidth`, but that feels quite hacky.

Locate the RichInlineItem definition and the existing extraWidth handling, then trace how inline element widths affect flow. Add support for a customWidth option for fixed-width custom emoji img elements, and verify that these elements flow naturally without a placeholder symbol or manual width subtraction while preserving existing behavior.

Written by the indexing model from the issue text.