types 3.0: breaking changes for block kit types

Taking inspiration from @seratch's library, https://github.com/seratch/slack-web-api-client/tree/main/src/block-kit, should study the following to see what kinds of improvements on the Block Kit side we could leverage in @slack/types.

Also, we should study bolt-js' use of composable event payloads and recursive Block Kit types, to see what improvements could land here. See the issues tagged in https://github.com/slackapi/node-slack-sdk/issues/1904

Idea: Add generics to layout blocks

What if we extended Blocks with generics, so that a containing layout block could constrain or fully specify the block elements contained within. An example on how this would look like for a dev:

const myImage: ImageElement = {
  type: 'image',
  alt_text: 'kitteh',
  image_url: 'https://kittens-r-us.com/kitteh.jpg',
}
const myContextBlock: ContextBlock<ImageElement> = {
  type: 'context',
  elements: [myImage], // compiles fine
}
const nopeBlock: ContextBlock<ImageElement> = {
  type: 'context',
  elements: [{ type: 'text', text: 'hello' }], // nope, TS complains that { type: 'text' } not assignable to ImageElement
}

The benefit here is more control and composability. Extending this further, we could employ a similar approach to other Slack domain objects and event payloads, so that they could all be composed in a similar way. For example, a block_actions event payload from a button click could be modeled by passing the ButtonElement into the BlockActionsEvent via generic (BlockActionsEvent<ButtonElement>), or the same idea for composing a View, and then passing that View into a ViewSubmissionEvent.. and so on.

Other Changes

• Define a utility type of Blocks based on availability in different Slack surfaces, see slack-web-api-client example

Start by comparing the Block Kit definitions in slack-web-api-client's src/block-kit/blocks.ts with the current @slack/types and review the related issues tagged from #1904. Then examine bolt-js's composable event payloads and recursive Block Kit types. Done should be a documented, agreed scope for the breaking type changes and the remaining utility-type work.