Add reusable test vectors

[KendallWeihe]

I've also added one example usage for decoding VC JWT, and had to fix something within the decode function

I originally wrote a test vector for did:jwk resolve but it was failing because we need to finish the work:

• Move from JsonWebKey2020 to JsonWebKey for DID VMs #121 (needs merging)
• Standardize DID types & add did:jwk test vector #120 (in a draft stage)

Will follow up with subsequent PRs for full coverage

[mistermoe]

he done it! he used the dashes! the thing everyone wants to do but shys away bc presumed go convention. thoughts on just testvectors.go? are you thinking we may end up adding moar vector loaders? if so, adding web5-spec def makes sense as prefix. alternatively, could use this file for loading all sorts of vectors and change the function name to LoadWeb5SpecVectors

[KendallWeihe]

oop, been in rust hacking land so forgetting golang conventions, change the filename!

thought about making the function specific to web5, but that data structure may be a standard or reusable across different vector sets, so leaving as-is for now

a bit annoying we have the relative file path in the test file but I can't think of an obvious better alternative

[mistermoe]

awesome stuff @KendallWeihe . really like the idea of a reusable vector loader