Move all the root styles to fragment files

Create assets/styles/root to hold all the fragments of the root
stylesheet. This should make it easier to find and hack on style. Yay!

assets/styles/root/050_syntax_highlighting.css

@@ -0,0 +1,20 @@
+.highlight code {
+    background-color: inherit;
+    margin-block: 0.5em;
+}
+
+.highlight .line {
+    display: grid;
+    grid-template-columns: max-content auto;
+}
+
+.highlight .ln a {
+    -webkit-user-select: none;
+    -moz-use-select: none;
+    -ms-user-select: none;
+    user-select: none;
+}
+
+.highlight .cl {
+    white-space: pre-wrap;
+}