fix(devtools): use visible gutter size in calculation for split area flex basis' (rangle/angular-devtools#857)

After PR rangle/angular-devtools#618, an invisible gutter was introduced to increase the draggable area of the split pane gutter. The gutter size being used in the calculation for the split area flex-basis still referenced the size of the now invisible gutter, causing split areas to be smaller than they should be. This is most noticable in the the input/output/state expandable panels in the property explorer tab.

Now the visible gutter size is used in the flex-basis calculation instead of the invisible one.

projects/ng-devtools/src/lib/vendor/angular-split/lib/component/split.component.ts

@@ -448,7 +448,10 @@ export class SplitComponent implements AfterViewInit, OnDestroy {
       }
       // Multiple areas > use each percent basis
       else {
-        const sumGutterSize = this.getNbGutters() * this.gutterSize!;
+        // Size in pixels
+        const visibleGutterSize = 1;
+        // Use visible gutter size in calculation instead of the invisible draggable gutter
+        const sumGutterSize = this.getNbGutters() * visibleGutterSize;
 
         this.displayedAreas.forEach((area) => {
           area.component.setStyleFlex(