extensionUtils.js: add version comparer

Sometimes it is needed to know if the current version of Gnome Shell is equal or greater than a specific one to decide whether one path of code or another must be used. The old versionCheck() function can be used, but sometimes a more generic comparison is needed for this.

This MR proposes a function to do that: it receives two strings with versions, and returns whether one is, or not, smaller than the other.