DUX-3104: expose the block kit builder

[lf-]

This was used in tests and is quite useful for relatively ergonomically writing block kit stuff.

Before submitting your PR, check that you've:

• Documented new APIs with Haddock markup
• Added @since declarations to the Haddock

After submitting your PR:

• Update the Changelog.md file with a link to your PR
• Bumped the version number if there isn't an (unreleased) on the Changelog
• Check that CI passes (or if it fails, for reasons unrelated to your change, like CI timeouts)

[linear]

DUX-3104 slack-web needs to expose the block builder to clients

[9999years]

sick