Rename jss&css folder to avoid path conflicts

[PTheocharis]

The folder jss&css contains the symbol '&', which is used during runtime to chain commands and it may cause disruptions and conflicts when there is a need to use paths.

Suggestion: Change it to js_css or jsCss (camelized option from the core.js), change all mentions with ctrl+F

[ImprovedTube]

(but it looks good?) (and we have yet to find a use case of conflict.) (and the price renaming costs, is adding an extra page to to edit histories on github to each of the files)

[PTheocharis]

It is more of a precaution measure or a suggestion I noticed, because I had to change some commands, while parsing :)