chore: start testing on Node.js 12 and 13
chore: start testing on Node.js 12 and 13

- make sure we're aware of all compatibility issues

handlebars-lang/handlebars.js,handlebars-lang/handlebars.js,mosoft521/handlebars.js,handlebars-lang/handlebars.js,mosoft521/handlebars.js,mosoft521/handlebars.js,gautamkrishnar/handlebars.js,handlebars-lang/handlebars.js,gautamkrishnar/handlebars.js,mosoft521/handlebars.js,mosoft521/handlebars.js,handlebars-lang/handlebars.js,gautamkrishnar/handlebars.js