[Fix] jsx-key not detecting missing keys in Array.from's mapping function

[sjarva]

Closes #3368

Now the jsx-key rule will catch these invalid cases:

Array.from([1, 2 ,3], function(x) { return <App /> });
Array.from([1, 2 ,3], (x => { return <App /> }));
Array.from([1, 2 ,3], (x => <App />));

and will allow these valid cases:

Array.from([1, 2, 3], function(x) { return <App key={x} /> });
Array.from([1, 2, 3], (x => <App key={x} />));
Array.from([1, 2, 3], (x => {return <App key={x} />}));
Array.from([1, 2, 3], someFn);
Array.from([1, 2, 3]);

In addition, I did some refactoring to keep the code DRY (the checking for keys in Array.prototype.map and Array.from had many repetitions).

[codecov]

Codecov Report

Merging #3369 (d826522) into master (b0d0ca1) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head d826522 differs from pull request most recent head bb4d1b3. Consider uploading reports for the commit bb4d1b3 to get more accurate results

@@           Coverage Diff           @@
##           master    #3369   +/-   ##
=======================================
  Coverage   97.55%   97.55%           
=======================================
  Files         123      123           
  Lines        8918     8928   +10     
  Branches     3255     3256    +1     
=======================================
+ Hits         8700     8710   +10     
  Misses        218      218           

Impacted Files	Coverage Δ
lib/rules/jsx-key.js	97.95% <100.00%> (+0.23%)	⬆️

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[ljharb]

Looks great, thanks!