Fire im_open event for the bot

[TK95]

Summary

This PR partially relates to #236.

Consider the use case:
Show a list of supported slash commands, messages, etc when the user opens IM with the bot for the very first time.

In order to implement that, you have to subscribe to the im_open event, but this event isn't fired for your bot.

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.

[codecov]

Codecov Report

Merging #331 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #331   +/-   ##
=======================================
  Coverage   73.71%   73.71%           
=======================================
  Files           7        7           
  Lines         506      506           
  Branches      145      145           
=======================================
  Hits          373      373           
  Misses        101      101           
  Partials       32       32

Impacted Files	Coverage Δ
src/middleware/builtin.ts	68% <ø> (ø)	⬆️

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[TK95]

It looks like, it's better to utilize the app_hope_opened event, it's fired whenever you open DM with your bot.

Closing PR.