Make --trace-extension-list-line-height a mappable CSS variable

The CSS variable --trace-extension-list-line-height is used in both the
vscode and theia trace extension. This commit maps this CSS variable to
--trace-viewer-trace-extension-list-line-height so each extension can
declare its own style.

Signed-off-by: Hoang Thuan Pham <hoang.pham@calian.ca>

packages/react-components/style/trace-explorer.css

@@ -1,5 +1,5 @@
 :root {
-    --trace-extension-list-line-height: 16px;
+    --trace-extension-list-line-height: var(--trace-viewer-trace-extension-list-line-height);
 }
 
 #trace-explorer-views-widget,

theia-extensions/viewer-prototype/style/trace-viewer.css

@@ -44,4 +44,5 @@
     --trace-viewer-sideBarSectionHeader-background: var(--theia-sideBarSectionHeader-background);
     --trace-viewer-view-container-title-height: var(--theia-view-container-title-height);
     --trace-viewer-sideBarSectionHeader-foreground: var(--theia-sideBarSectionHeader-foreground);
+    --trace-viewer-trace-extension-list-line-height: 16px;
 }