Various extensions to JSONValue

[patrickfreed]

This PR adds the following extensions to the JSONValue enum:

• Conformances to various ExpressibleByXLiteral protocols (so e.g. let x: JSON = ["y": [ "z": [1, 2.2, false]]] is possible)
• Conformance to the Codable protocol (if you want a loosely typed value as part of your Codable struct, for example)
• Computed properties for failably unwrapping Swift types

[codecov[bot]]

Codecov Report

Merging #68 (1b1c179) into main (122b945) will increase coverage by 0.28%. The diff coverage is 84.52%.

@@            Coverage Diff             @@
##             main      #68      +/-   ##
==========================================
+ Coverage   85.91%   86.19%   +0.28%     
==========================================
  Files          13       13              
  Lines        1271     1355      +84     
==========================================
+ Hits         1092     1168      +76     
- Misses        179      187       +8     

Impacted Files	Coverage Δ
Sources/ExtrasJSON/JSONValue.swift	91.41% <84.52%> (-3.89%)	:arrow_down:
...ON/Decoding/JSONSingleValueDecodingContainer.swift	100.00% <0.00%> (+3.17%)	:arrow_up:
...ON/Encoding/JSONSingleValueEncodingContainer.swift	68.51% <0.00%> (+5.55%)	:arrow_up:

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