[FEATURE] expose block types

[eikooc]

I'd like to have the block types exposed just like the NPM module:

For example with the nodejs module:

import type {
  ContextBlock,
  DividerBlock,
  HeaderBlock,
  SectionBlock,
} from 'npm:@slack/[email protected]'

I don't think the types are exposed in the deno module

Description of the problem being solved

Types for creating a chat request exposed.