revert: "refactor: Clean up golden-layout css (#1322)" (#1334)

This reverts commit d1838d7.

Was getting build errors when consuming in Enterprise:
```
> Task :web-client-ui:ideClient FAILED
✓ 1043 modules transformed.
Could not resolve '../scss/GoldenLayout.module.css' from node_modules/@deephaven/golden-layout/dist/GoldenLayoutThemeExport.js
error during build:
Error: Could not resolve '../scss/GoldenLayout.module.css' from node_modules/@deephaven/golden-layout/dist/GoldenLayoutThemeExport.js
    at error (file:///Users/bender/dev/illumon/iris/web/client-ui/node_modules/rollup/dist/es/shared/rollup.js:1858:30)
    at ModuleLoader.handleResolveId (file:///Users/bender/dev/illumon/iris/web/client-ui/node_modules/rollup/dist/es/shared/rollup.js:22156:24)
    at file:///Users/bender/dev/illumon/iris/web/client-ui/node_modules/rollup/dist/es/shared/rollup.js:22119:26
ERROR: "build-js" exited with 1.
```

In the `dist/GoldenLayoutThemeExport.js`, the import ends up being:
```
import GoldenLayout from "../scss/GoldenLayout.module.css"; // parseInt for unitless values, stripping "px"
```
But in the scss folder it’s `GoldenLayout.module.scss`, not `.css`

The babel preset converts the scss imports to css:
```
    [
      'transform-rename-import',
      {
        // The babel-plugin-add-import-extension adds the .js to .scss imports, just convert them back to .css
        original: '^(.+?)\\.s?css.js$',
        replacement: '$1.css',
      },
    ],
```

Will need to resolve this issue before re-merging this refactor.

Author: mofojed

package-lock.json

@@ -4,13 +4,9 @@
   display: flex;
   flex-direction: column;
   justify-content: center;
+  padding: $spacer-3;
   min-height: 100%;
   max-height: 100%;
   text-align: center;
   overflow: hidden;
-
-  // add padding on an inner wrapper so it is included in max height of parent
-  div {
-    padding: $spacer-3;
-  }
 }

packages/dashboard/src/PanelPlaceholder.scss

@@ -11,7 +11,7 @@ const PanelPlaceholder = React.forwardRef(
     const component = LayoutUtils.getComponentNameFromPanel({ props });
     return (
       <div ref={ref} className="panel-placeholder">
-        <div>Component &quot;{component}&quot; is not registered.</div>
+        Component &quot;{component}&quot; is not registered.
       </div>
     );
   }

packages/dashboard/src/PanelPlaceholder.tsx

@@ -0,0 +1,9 @@
+@import '@deephaven/components/scss/custom.scss';
+@import './GoldenLayout.scss';
+
+// Exported variables for use in GoldenLayoutThemeExport.js
+/* stylelint-disable property-no-unknown */
+:export {
+  tab-height: $tab-height;
+  drag-border-width: $drag-border-width;
+}