var->let (or const)

[Toine-ddt]

in general prefer let (and const when it makes sense) to var:

• https://dev.to/kunalt96/var-let-const-in-javascript-3dne
• https://medium.com/podiihq/javascript-variables-should-you-use-let-var-or-const-394f7645c88f
• https://babeljs.io/docs/en/learn#let--const

_Originally posted by @jcelerier in https://github.com/ossia/qml-remote/pull/82#discussion_r667717086_