Improve styling of column resize handles

- Only show resize divider on table header
- Use same color for resize divider as for dividers of data
- Add bottom border on header to better match overall style

Closes #71

media/memory-table.css

@@ -14,6 +14,7 @@
  * SPDX-License-Identifier: EPL-2.0 OR GPL-2.0 WITH Classpath-exception-2.0
  ********************************************************************************/
 
+.memory-inspector-table thead th,
 .memory-inspector-table .group-separator > td {
   border-bottom: 2px solid var(--vscode-editor-lineHighlightBorder);
 }
@@ -83,3 +84,24 @@
 .radix-prefix {
   opacity: .6;
 }
+
+/* == Resize Handle == */
+
+.memory-inspector-table span.p-column-resizer:before {
+  content: '';
+  position: absolute;
+  border-right: 2px solid var(--vscode-editor-lineHighlightBorder);
+  z-index: 0;
+  transform: translateX(-50%); /* center of span */
+  height: 100%;
+  width: 100%;
+  transition: border-right .1s ease-out;
+}
+
+.memory-inspector-table span.p-column-resizer:hover:before {
+  border-right: 2px solid var(--vscode-sash-hoverBorder);
+}
+
+.memory-inspector-table .p-column-resizer-helper {
+  width: 2px;
+}