MC7000: Fix code style issues in the controller script

[fwcd]

There are a bunch of style issues in the script that the pre-commit hook didn't accept, mostly regarding const/let declarations of variables.

[toszlanyi]

Const/let its not working with the current releases of Mixxx, only for the future. So all your improvements will not be able to run on current release. Just my thought but I guess, the shift and jog acceleration should go first, so based to 2.3 branch and the const/let change to 2.4 or main branch. Maybe @Swiftb0y can comment better on this topic. Cheers

[uklotzde]

Fixing the ES6 linter issues on main should be deferred until no more 2.3.x releases are expected. Otherwise this would cause serious merge conflicts when improvements or fixes for 2.3.x mappings are provided. In this case #4729 would be affected.

[github-actions]

This PR is marked as stale because it has been open 90 days with no activity.

[fwcd]

Looks like the 2.3 merges (e.g. 17f3c42 and 302b40e) already took care of everything in this PR (I think the code style changes were introduced during these merges since I couldn't find more granular commits).