Add JSON utils for returning kinds, valid utf8, and kinds to strings

[davekay100]

Found these here: https://github.com/nanspro/kauri-subgraph/blob/master/src/util.ts

• Untested. Opening the PR to make us aware of it
• I am found of them all, except kindToString() - not too sure if this is useful enough to be in the library
• isValidUTF8() looks great, but I am unaware of how it currently works
• Would be nice to add in for asBigInt() and asI32()

What do you guys think? @Jannis @leoyvens

[leoyvens]

The as* APIs for JSON seem useful, but they should be put directly under JSONValue.