[v2] Suggest string literals on IDE when typing the kind of conversation in <ConversationsSelect include>

[yhatt]

include prop of <ConversationsSelect> allows string type, to accept the space-separated kind of conversations. jsx-slack v1 has suggested the kind while typing array value through <Conversations include={[]}>, but not while typing a string.

I've improved its type definition for auto-suggestion on IDE to work the suggestion in string.

type FilterIncludeKind = 'im' | 'mpim' | 'private' | 'public'
type SpaceSeparatedKind = string & { _?: never }

interface FilterProps {
  include?: FilterIncludeKind | FilterIncludeKind[] | SpaceSeparatedKind
}

This hack is called as LiteralUnion in microsoft/TypeScript#29729 (comment).