Recommend the use of Less namespaces for extensions

To prevent any mixin collisions between extensions, we should recommend the use of Less namespaces, similar to how we handle initiali(s/z)ers in JS.

https://lesscss.org/features/#mixins-feature-namespaces

They're not _that_ tricky to understand, thankfully, using an ID-based system.

My suggested syntax would be `#vendor__package`. I don't know how Less would handle slashes, etc.

```less
#davwheat__sidebar-nav-items {
.my-cool-mixin() {
color: red;
}
}

button.davwheat_btn {
#davwheat__sidebar-nav-items.my-cool-mixin();
}
```

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Eventually, core should switch to something like `#flarum__core.xxx()`, or similar.