Automatically add cache bust query string to icons.woff

MR ready for a review.

The whole thing is fit into the current theme build process, so nothing is changed there. The step package.json `build-icons` step now use execute `node build-icons.js` and all hash calculation and update of needed files is done there. Hash is injected as a SASS variable - with existing $bs-icon-font-hash variable, and it is also injected into theme info yaml file if icons font asset preloading is used.

I considered couple of approaches for this but on the end I sticked to this solution because this is how we currently update are icons assets - child themes icons and build process is separated from parent theme icons and build process.

New update function `bs_base_bs_update_200003()` should update child themes to this new hash fully so no manual update process is needed.

pivica → credited berdir → .

@berdir provided couple of feedbacks, all are addressed, this is merged.