Drop Node.js 12.x/14.x support, update tooling.

[davidlehn]

• BREAKING: Remove support for Node.js 12.x and 14.x. This is done to allow updates to tooling that no longer support older Node.js versions.
• Update development dependencies.
• Add karma testing.
  • Use karma-server-side PR fork hash to support modern karma.
• Update keywords.
• Move test/benchmark env docs into common test.js.
• Disable native lib use when testing in a browser.
• Disable sync tests in a browser. The sync code attempts to use the async webcrypto calls and produces invalid results. It is an error that this doesn't fail, but sync code is currently only for testing.
• Only load rdf-canon tests once.

[codecov-commenter]

Codecov Report

Merging #61 (1b8e4c2) into main (0d893ad) will decrease coverage by 27.20%. The diff coverage is n/a.

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@@             Coverage Diff             @@
##             main      #61       +/-   ##
===========================================
- Coverage   88.77%   61.57%   -27.20%     
===========================================
  Files           8        8               
  Lines         570      570               
===========================================
- Hits          506      351      -155     
- Misses         64      219      +155     

see 2 files with indirect coverage changes

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