Split up View type to improve TS linting

[alexjamesmalcolm]

Summary

There a View type. It has three subset types modal, home, and workflow_step. As I've discovered while consuming @slack/bolt there is a benefit to split these three up. Specifically that the modal subset requires the title field but the other two subsets have no use for it. If a consumer of this attempts to send a modal view without a title they will receive an error so by splitting up the single interface into three we can prevent the consumer from having to make the same mistake as myself to learn that although the title field is optional for a view, when type: "modal" is the case the title field is required.

I had to rely on these two pieces of documentation to get all of the fields correct:

• workflow_step docs
• home and modal docs

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[CLAassistant]

All committers have signed the CLA.

[seratch]

Thanks for the great pull request! Can you check my comment?

[seratch]

LGTM! Before merging this, I would like to wait for others' comments for a few days

[srajiang]

I think we should be good to merge these changes in, unless there was further discussion to be had on this @seratch!

[seratch]

Thanks @srajiang, yes, we can merge this PR 👍