Use Guild member counts rather than user cache size

[Lioness100]

This is mostly a fix for discord.js v12+

Currently, DiscordJS#userCount relies on the user cache, which will result in inaccurate numbers. (Much) more often than not, the user cache will not include every single user. Instead, it would be better to use the guild cache (since by default they're all guaranteed to be there) using Collection#reduce() and Guild#memberCount

[EndBug]

Hi, thanks for the PR!
At the moment I'm waiting to marge a PR that changes how the lib folder is handled, so we'll need to update this PR after I merge that.

Anyway, regarding your contribution: the change looks good to me. The tests for Poster#post are failing, the dummy client should be updated to fit the new way of getting the user count.

[Snazzah]

This change should be mirrored to the other client fillers if possible. I'm pretty sure the other client fillers rely on user cache rather than member count.

[codecov]

Codecov Report

Merging #182 (7a43b33) into master (d55d45c) will decrease coverage by 0.56%.
The diff coverage is 50.00%.

❗ Current head 7a43b33 differs from pull request most recent head 014fca0. Consider uploading reports for the commit 014fca0 to get more accurate results

@@            Coverage Diff             @@
##           master     #182      +/-   ##
==========================================
- Coverage   96.98%   96.42%   -0.57%     
==========================================
  Files          45       45              
  Lines        1196     1204       +8     
  Branches      191      191              
==========================================
+ Hits         1160     1161       +1     
- Misses         32       39       +7     
  Partials        4        4              

Impacted Files	Coverage Δ
src/Interface/ClientFiller.ts	100.00% <ø> (ø)
src/Interface/Clients/DiscordIO.ts	70.00% <25.00%> (-7.78%)	⬇️
src/Interface/Clients/Eris.ts	88.23% <50.00%> (-5.52%)	⬇️
src/Interface/Clients/Paracord.ts	86.66% <50.00%> (-13.34%)	⬇️
src/Interface/Clients/DiscordJS.ts	81.81% <60.00%> (-3.19%)	⬇️
src/Interface/Clients/Discordie.ts	87.50% <66.66%> (-5.84%)	⬇️

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[EndBug]

@Snazzah Maybe it's better if we merge this PR and change only discord.js for now.
Then we can always change the other ones too, but maybe it's better if you do it, I'm not really that familiar with the other libraries.

[Lioness100]

This looks great!

[EndBug]

Awesome!

[EndBug]

@Lioness100 All set then! Thanks again for your contribution! The fix will be published in the next version 👍🏻