refactor(client): Scope global CSS selectors in Debug panel (#527)

Previously, `<button>`, `<section>`, and `<select>` elements where being 
styled directly with global scope. This poses a fairly high risk of 
leaking styles to elements in the rest of a game using the debug panel.

This restricts the scope of those global selectors to only apply to 
children of the root debug panel component.

src/client/debug/Debug.svelte

@@ -74,7 +74,7 @@
     background: #fefefe;
   }
 
-  :global(button, select) {
+  .debug-panel :global(button, select) {
     cursor: pointer;
     outline: none;
     background: #eee;
@@ -84,21 +84,21 @@
     border-radius: 3px;
   }
 
-  :global(button) {
+  .debug-panel :global(button) {
     padding-left: 10px;
     padding-right: 10px;
   }
 
-  :global(button:hover) {
+  .debug-panel :global(button:hover) {
     background: #ddd;
   }
 
-  :global(button:active) {
+  .debug-panel :global(button:active) {
     background: #888;
     color: #fff;
   }
 
-  :global(section) {
+  .debug-panel :global(section) {
     margin-bottom: 20px;
   }
 </style>