feat(icons): add `ux:icons:lock` command to "mass import" used icons

[kbond]

Q	A
Bug fix?	no
New feature?	yes
Issues	n/a
License	MIT

There has been valid concern over using the on-demand system in production. Icons could change/be removed or the iconify service could be down. This new command searches your twig files and imports valid iconify.design icons in your twig file and mass imports them locally. By default, they aren't overwritten but using the --force option enables this.

I think, the official best practice should be:

• on-demand enabled in development only
• part of your workflow includes running ux:icons:lock (could be automated)
• (future) ux:icons:lint to be used in your CI to ensure all icons are available locally

This could/should be reflected in the official recipe (TODO).

[smnandre]

So if i understand:

• during dev, you use ux_icon(foo), foo is magically downloaded from iconify and stored in cache
• if you call the lock method, it's saved in assets/icons (and can be committed)
• if you call another time lock with --force you can override the local one

This seem really sensible!

(+1 for the lint command idea)