v2.0.0

• Improved type safety for Slack identifiers
• Slack header block support in DSL (https://api.slack.com/reference/block-kit/blocks#header)
• Updated deps: Sttp, FS2
• Removed support for legacy/deprecated API

Migration Notices
Before v2.0.0 most of Slack identifiers data types were String. To make your code safer and improve the whole type-safety, now they all have specialised classes instead of using general String (like SlackChannelId, etc).