Add interface for default_to_current_conversation field on conversation selects

[yhatt]

Today Slack has added default_to_current_conversation field into (multi_)conversation_select menu.
https://api.slack.com/changelog#April_2020

A more direct route to messaging in response to modals: the new default_to_current_conversation field allows your conversation_select and multi_conversation_select menus to be pre-populate the currently open conversation.

Proposal

We have already a lot of props for <ConversationSelect> and have to make a simple interface if possible.

The added field is mutually exclusive with initial_conversations, so I think that to accept current special string in initialConversation prop (or aliased value prop) is better.

<Modal title="test">
  {/* Pre-select a conversation that opened modal from. */}
  <ConversationsSelect label="Send to..." initialConversation="current" />
</Modal>

At the present time, multi_conversations_select does not seem to be able to mix the current conversation along with specific conversations. Probably <ConversationsSelect multiple value={['current', 'C0123456789']} /> is valid as Block Kit JSON, but Slack API will ignore the state of default_to_current_conversation generated from current value.