<RichText> block component

[yhatt]

https://api.slack.com/reference/block-kit/blocks#rich_text

rich_text block becomes able to use in Slack API. It has a similar rendering with <Section> but the output JSON is much structed. There are some advantages:

• It has the same structure as messages by Slack user
• Can use the actual list items, not mimicked
• It does not need to escape mrkdwn special characters, and may give a solution for a long standing issue: Escaped special chars in Android have incorrect rendering  #187

In jsx-slack, it requires new implementation of HTML parser for rich_text block format.
https://api.slack.com/changelog/2019-09-what-they-see-is-what-you-get-and-more-and-less