Fix/const to var

[mllustosa]

This is related to the issue #14380. The latest update used const variable declaration which is not compatible with UglifyJS (UglifyJS doesn't support ES6). From what I saw, those were the only places where the library is using const and in all other places var is used, so I assumed that it was by mistake.

Of course that this is just a small draft and you guys know better what needs to be changed and where, but I thought that giving some sort of direction wouldn't hurt

I understand that:

• I'm submitting this PR for reference only. It shows an example of what I'd like to see changed but
  I understand that it will not be merged and I will not be listed as a contributor on this project.

[clark-stevenson]

Thanks for this @mllustosa

I confirm that my webpack build is failing at the uglifyjs step because of it. I was in the middle of writing a stack overflow about why const was ending up in my output which I didn't expect.

[tagliala]

@robmadole has this been fixed in 5.6.1?

[robmadole]

Yeah we've fixed this in 5.6.1 👍

[tagliala]

@mllustosa thanks for your PR

[mllustosa]

@tagliala Anytime. Thanks for this awesome library and for the great and quick support 😄