Adding HtmlWebpackPlugin@4 support

[AnujRNair]

Summary

Building off of PR #16, this PR adds HtmlWebpackPlugin@4 support to the CspHtmlWebpackPlugin.

Opted to move the require to the top of the file to ensure we load it once and reference it - also keeps the requires a little more together for easier debugging.

Credits go to @sibiraj-s for the initial implementation in the PR linked above

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.
• I've written tests to cover the new code and functionality included in this PR.
• I've read, agree to, and signed the Contributor License Agreement (CLA).

[codecov]

Codecov Report

Merging #23 into master will decrease coverage by 1.56%.
The diff coverage is 62.5%.

@@            Coverage Diff             @@
##           master      #23      +/-   ##
==========================================
- Coverage   98.88%   97.32%   -1.57%     
==========================================
  Files           2        2              
  Lines         180      187       +7     
  Branches       12       14       +2     
==========================================
+ Hits          178      182       +4     
- Misses          2        5       +3

Impacted Files	Coverage Δ
plugin.js	93.05% <62.5%> (-3.87%)	⬇️

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