Add sectionTitle prop to <Mention> for grouped suggestion headers

[RonenMars]

Add functionality of displaying sections inside the component.

Preview:

Functionally:
When multiple <Mention> components share the same trigger, their suggestions are now displayed under labeled section headers in the dropdown. Pass sectionTitle to any <Mention> to opt in — if that group returns no results, its header is suppressed automatically.

<MentionsInput value={value} onChange={onChange}>
  <Mention trigger="@" data={colleagues} sectionTitle="Colleagues" />
  <Mention trigger="@" data={bots} sectionTitle="Bots" />
</MentionsInput>

The header is styled via the sectionTitle key in the style prop, consistent with how all other parts of the overlay are styled.

[changeset-bot]

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Latest commit: b7b2b5e

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[hbmartin]

@RonenMars I don't think this project is maintained, would you consider opening a PR for https://github.com/hbmartin/react-mentions-ts ?