tools: overhaul tools/doc/json.js

Modernize:
* Replace `var` with `const` / `let`.
* Wrap `switch` cases with `const`/`let` in blocks.
* Replace common functions with arrow functions.
* Replace string concatenation with template literals.
* Shorthand object literals.
* Use destructuring and spread.

Optimize:
* Move RegExp declaration out of loops.
* Replace `.match()` with `.test()` in boolean context.
* Replace RegExp with string when string suffices.
* Make RegExp more strict to reject unrelated cases.
* Make RegExp do the trimming to eliminate many `.trim()` calls.
* Cache retrieved object properties.
* Remove conditions that cannot be false.
* Remove code that seems obsolete
  (it means a state that cannot happen or is not typical).

Clarify:
* Sync code examples in comments with the actual source state.
* Expand some one-letter variable names.
* Rename confusingly similar variables.
* Move variable declarations closer to their context.
* Remove non-actual commented out code.
* Unify blank lines betw...