Enable no-shadow ESLint rule

[alex-ketch]

A tangential note for the JS, I think we should enable the ESLint no-shadow rule to avoid bugs and simplifying process for reading code.

Originally posted by @alex-ketch in stencila/thema#84

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Motivation: From the rule reference

Shadowing is the process by which a local variable shares the same name as a variable in its containing scope. For example:

var a = 3;
function b() {
   var a = 10;
}

In this case, the variable a inside of b() is shadowing the variable a in the global scope. This can cause confusion while reading the code and it's impossible to access the global variable.

[alex-ketch]

Reverting this change in 3cd10c6 due to large number of false positive errors
See eslint/eslint#12893

[alex-ketch]

New issue to track issue with false positives: eslint/eslint#14963