fix: improve accessible page structure

[nolanlawson]

Makes some progress on #294

• Remove emojilist i18n text and region, just let each category have its own region
• Add a region for the entire emoji picker, same as the title shown in the preview bar
• Add a region for the search, using the same label as the placeholder for the input
• Make the preview aria-hidden since it is decorative

This may still be better with headers rather than regions, but since we can't know how emoji-mart will be hosted on a page, we can't know whether to use h1 vs h2, etc. So regions seem safer to me, but admittedly I'm not an expert.