fix: replace const with var to avoid strict mode errors

[MikeyParton]

This PR contains a:

• bugfix
• new feature
• code refactor
• test update
• typo fix
• metadata update

Motivation / Use-Case

I'm using this plugin on an app running on some old webkit browsers (~Safari 7). After updating to webpack 5, I get the following error when running in development mode.

SyntaxError: Unexpected keyword 'const'. Const declarations are not supported in strict mode.

All the surrounding template code is using var, this one must have been missed.

Breaking Changes

None

Additional Info

None

[codecov]

Codecov Report

Merging #735 (c0d7b49) into master (e8918ab) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #735   +/-   ##
=======================================
  Coverage   89.69%   89.69%           
=======================================
  Files           6        6           
  Lines         776      776           
  Branches      240      240           
=======================================
  Hits          696      696           
  Misses         77       77           
  Partials        3        3           

Impacted Files	Coverage Δ
src/index.js	88.03% <ø> (ø)

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[alexander-akait]

Big thanks