Update code to use `let` and `const`

[postmodern]

Update the code to use let and const instead of var, and other modern ECMAScript practices.

[daniloff200]

Hey, @postmodern, is it still an actual one? I can pick that I think, I'm usually working with Angular and node.js/nest.js so can do that

Can this be counted as a Hacktoberfest one?

[postmodern]

I plan on refactoring all of the ronin-agent-* repos, but don't have the time currently. Before making any changes, this repo also needs tests (see issue #1). If you want to work on this repo, I might be willing to add the hacktoberfest tag to it.