Message Svelte component - don't remove text slot from build

[hjkl]

I'd like to use the text slot defined on the Message component (to add sanitized html with {@html}

Unfortunately, build-svelte-types removes the 'text' slot from the Message component's type definitions as the line doesn't contain React.ReactNode

Added this to the slot and works correctly on the component for Svelte - unsure what the ramifications are for React.