✨ (bootsier): Añade AdminLTE 4 y Bootstrap Icons

Integra AdminLTE 4.0.0 (SCSS y JS), Bootstrap Icons 1.13.1 y las fuentes
Source Sans 3 VF; reorganiza los SCSS del tema con la nueva estructura
de *pagetop-build*.

extensions/pagetop-bootsier/assets/bootstrap-5.3.8/scss/_transitions.scss

@@ -0,0 +1,27 @@
+.fade {
+  @include transition($transition-fade);
+
+  &:not(.show) {
+    opacity: 0;
+  }
+}
+
+// scss-docs-start collapse-classes
+.collapse {
+  &:not(.show) {
+    display: none;
+  }
+}
+
+.collapsing {
+  height: 0;
+  overflow: hidden;
+  @include transition($transition-collapse);
+
+  &.collapse-horizontal {
+    width: 0;
+    height: auto;
+    @include transition($transition-collapse-width);
+  }
+}
+// scss-docs-end collapse-classes