Clarify ECMAScript version(s) supported

[stu002]

Many thanks for providing the ECMAScript scripting interface.

For QCAD users working with the ECMAScript scripting interface it would be helpful to quickly determine in the documentation and the scripting window the currently supported language version.

ES2015, ES2016, ES2017, ES2018, and ES2019 versions have been standardized and offer very useful language and library additions over earlier language versions. Knowing the supported language version will help integrate QCAD scripts with other Javascript tooling that supports multiple language versions.

[stu002]

For QCAD users working with the ECMAScript scripting interface it would be helpful to quickly determine in the documentation and the scripting window the currently supported language version.

It looks like the ECMAScript version may be unknown or undefined so I think we need to assume it's ES2015 and use a transpiler to access more modern language features.