Detect BigInt within circular() and properly convert within JSON.stringify

[rdmurphy]

Closes #175.

I used the more complex check of Object.prototype.toString.call(val) === '[object BigInt]' instead of just doing typeof val === 'bigint' to account for anyone doing something absurd like Object(2n) or Object(BigInt(3) just to cover all the bases.

Also may kinda smell that this is happening within circular(), but I didn't want to do anything more expansive than that. 😅

I followed the lead of MDN and used toString() as the JSON-friendly output. When you run console.log on a BigInt it prints it with a trailing n to signify it is a special value. I could maybe see doing that instead for clarity? They'd still have to be a string though so the effect isn't entirely the same.

["1", "2", "3", "4"]
// vs.
["1n", "2n", "3n", "4n"]

[codecov-commenter]

Codecov Report

Merging #186 (9eeaa98) into master (0761ef9) will decrease coverage by 0.60%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #186      +/-   ##
==========================================
- Coverage   89.48%   88.88%   -0.61%     
==========================================
  Files           4        3       -1     
  Lines         333      306      -27     
==========================================
- Hits          298      272      -26     
+ Misses         35       34       -1     

Impacted Files	Coverage Δ
src/diff.js	98.40% <100.00%> (+0.01%)	⬆️
src/parse.js
src/index.js	75.49% <0.00%> (+0.24%)	⬆️

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