refactor: change config file name

[ltsNotMike]

Ticket

Summary

What does this PR do/solve?

Changes misleading naming of the config file from "custom.js" to "plugins.js".

Test Plan

• Install plugin to strapi project.
• Add configuration
• Check if the configuration is being applied

[codecov-commenter]

Codecov Report

Merging #87 (3f720a0) into master (8cab133) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #87   +/-   ##
=======================================
  Coverage   14.36%   14.36%           
=======================================
  Files           5        5           
  Lines         348      348           
  Branches      111      111           
=======================================
  Hits           50       50           
  Misses        238      238           
  Partials       60       60           

Flag	Coverage Δ
unittest	14.36% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
services/utils/functions.js	47.27% <100.00%> (ø)

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