drop support for webpack 1

[danez]

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Please check if the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior? (You can also link to an open issue here)

What is the new behavior?

Does this PR introduce a breaking change?

• Yes
• No

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• Migration path for existing applications:
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Other information:

[codecov]

Codecov Report

Merging #407 into 7.0 will decrease coverage by -0.43%.
The diff coverage is 80%.

@@            Coverage Diff            @@
##              7.0    #407      +/-   ##
=========================================
- Coverage   83.03%   82.6%   -0.43%     
=========================================
  Files           6       6              
  Lines         165     161       -4     
  Branches       38      37       -1     
=========================================
- Hits          137     133       -4     
  Misses         12      12              
  Partials       16      16

Impacted Files	Coverage Δ
src/index.js	84.5% <80%> (-0.83%)	❌

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