[FEATURE] expose block types

I'd like to have the block types exposed just like the NPM module:

For example with the nodejs module:

```js
import type {
  ContextBlock,
  DividerBlock,
  HeaderBlock,
  SectionBlock,
} from 'npm:@slack/bolt@3.13.1'
```

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.