Update message types

[shaydewael]

Summary

Addresses #774 and updates other fields that were missing

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.

[shaydewael]

For context, I found the file type here: https://api.slack.com/types/file

[seratch]

As this field does not always exist, we may want to make it optional.

[seratch]

Also, attachments is missing.

[seratch]

Can you update the spec file to be compatible with the new type definition?

TSError: ⨯ Unable to compile TypeScript:
Error: src/middleware/builtin.spec.ts(752,7): error TS2322: Type '{ type: "message"; subtype: "bot_message"; channel: string; user: string; ts: string; text: string; bot_id: string; channel_type: "channel"; }' is not assignable to type 'MessageEvent'.
  Property 'event_ts' is missing in type '{ type: "message"; subtype: "bot_message"; channel: string; user: string; ts: string; text: string; bot_id: string; channel_type: "channel"; }' but required in type 'BotMessageEvent'.

Apart from this build failure, this PR looks great to me 👍

[seratch]

You can merge this once the build failure is fixed.

[seratch]

This branch still have a lint error. You need to apply code formatter to the modified spec files. Can you run npx prettier -w src/ && npm test and push the change?