Fixes an issue where `ue.getWordAtDocPos is not a function` would be thrown in `MessageInput.svelte`.
The error was caused by a timing issue where the `getWordAtDocPos` method on the `RichTextInput` component was not available when called from an event handler within the same component.
This change refactors the code to pass the `getWordAtDocPos` function as a callback prop from `RichTextInput` to `MessageInput`, ensuring it's available when needed.
7a80e60785