Fix update image block type

[cfeeney5]

Summary

This update adds the newly available slack_file object to the imageBlock and sets the image_url to be optional

Requirements (place an x in each [ ])

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

[salesforce-cla[bot]]

Thanks for the contribution! Before we can merge this, we need @cfeeney5 to sign the Salesforce Inc. Contributor License Agreement.

[seratch]

Thank you for taking the time to send this! As I mentioned at https://github.com/slackapi/node-slack-sdk/issues/1734#issuecomment-1911534468, a similar change is required on the image block element side. Also, we prefer union types for this.

[cfeeney5]

Ive refactored this now for use with unions. UrlImageObject and SlackFileImageObject are exported from block-elements so it can be used there and in blocks. Im not sure which way is preferred.

[filmaj]

I've enabled the CI tests for this PR and it seems the linter is now failing. Could you please also fix those up? I think many of the formatting changes you introduced will not be allowed according to our linter configuration / style guide.

[filmaj]

#1783 has superseded this PR; closing.

[filmaj]

FYI the fix for this has been released to npm as @slack/types v2.12.0.