Add did key error codes

[aljones15]

• Adds a new file ./lib/validators.js
• Adds a new validator `checkKeyBytes.
• Separates TypeError from SyntaxError
• DRYs up key bytes checks
• Add error code for invalidPublicKeyLength to public key checks.

#18

[codecov-commenter]

Codecov Report

Merging #19 (8537fcd) into main (56eb921) will decrease coverage by 0.36%.
The diff coverage is 78.18%.

@@            Coverage Diff             @@
##             main      #19      +/-   ##
==========================================
- Coverage   92.01%   91.65%   -0.37%     
==========================================
  Files           3        4       +1     
  Lines         614      659      +45     
==========================================
+ Hits          565      604      +39     
- Misses         49       55       +6     

Impacted Files	Coverage Δ
lib/validators.js	62.50% <62.50%> (ø)
lib/Ed25519VerificationKey2020.js	92.09% <100.00%> (+0.12%)	⬆️
lib/ed25519.js	96.12% <100.00%> (+4.18%)	⬆️

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[dmitrizagidulin]

👍 this is great

[msporny]

First pass seems fine to me... I expect @dlongley might have more specific thoughts on the structure of the call... for example, do we expect ALL key libraries to export an assertKeyBytes method? When would you use that vs. checkKeyBytes? It was not immediately evident what the answers to these questions were.