feat(toc): implement post table of contents and refactor line number component

src/styles/post-line-num.scss

@@ -0,0 +1,54 @@
+@import "variables";
+
+.post-line-gutter {
+  position: absolute;
+  left: -80px;
+  top: 0;
+  width: 60px;
+  user-select: none;
+  opacity: 0.5;
+  border-right: 1px solid color-mix(in srgb, var(--foreground) 15%, transparent);
+
+  @media (max-width: $tablet-max-width) {
+    display: none;
+  }
+
+  .line-number {
+    position: absolute;
+    right: 8px;
+    display: flex;
+    align-items: center;
+    justify-content: flex-end;
+    padding-right: 10px;
+    font-size: 0.85rem;
+    color: var(--foreground);
+    transition: opacity 0.15s ease;
+    opacity: 0.8;
+
+    &:hover {
+      opacity: 1;
+    }
+  }
+
+  .line-number--gap {
+    opacity: 0.5;
+  }
+
+  .line-number--breakpoint {
+    cursor: pointer;
+
+    .line-breakpoint {
+      position: absolute;
+      right: -3px;
+      color: var(--brightRed);
+      font-size: 0.875rem;
+      font-family: Hack, Monaco, Consolas, monospace;
+      transition: transform 0.15s ease, filter 0.15s ease;
+    }
+
+    &:hover .line-breakpoint {
+      transform: scale(1.2);
+      filter: brightness(1.2);
+    }
+  }
+}