[memoryprofiler] Add CSS class to parent div (emscripten-core#25595)

Add a CSS class to the main parent div of the memory profiler UI.

Currently, the profiler's container div is created with no ID or class,
making it impossible to target with CSS. This is problematic for
developers (like in the original issue) who run their application in
full-screen, as the profiler UI gets hidden with no way to adjust its
`z-index`.

This change adds the class `emscripten-memory-profiler-container` to the
div upon its creation in `library_memoryprofiler.js`.

This allows developers to easily style the profiler. For example:

```
<style>
  .emscripten-memory-profiler-container {
    z-index: 9999;
  }
</style>
```

Fixes: emscripten-core#20115

Author: shreyas-omkar

src/memoryprofiler.js

@@ -272,6 +272,7 @@ var emscriptenMemoryProfiler = {
     var div;
     if (!emscriptenMemoryProfiler.memoryprofiler_summary) {
       div = document.createElement("div");
+      div.className = 'emscripten-memory-profiler-container';
       div.innerHTML = "<div style='border: 2px solid black; padding: 2px;'><canvas style='border: 1px solid black; margin-left: auto; margin-right: auto; display: block;' id='memoryprofiler_canvas' width='100%' height='50'></canvas><input type='checkbox' id='showHeapResizes' onclick='emscriptenMemoryProfiler.updateUi()'>Display heap and sbrk() resizes. Filter sbrk() and heap resize callstacks by keywords: <input type='text' id='sbrkFilter'>(reopen page with ?sbrkFilter=foo,bar query params to prepopulate this list)<br/>Track all allocation sites larger than <input id='memoryprofiler_min_tracked_alloc_size' type=number value="+emscriptenMemoryProfiler.trackedCallstackMinSizeBytes+"></input> bytes, and all allocation sites with more than <input id='memoryprofiler_min_tracked_alloc_count' type=number value="+emscriptenMemoryProfiler.trackedCallstackMinAllocCount+"></input> outstanding allocations. (visit this page via URL query params foo.html?trackbytes=1000&trackcount=100 to apply custom thresholds starting from page load)<br/><div id='memoryprofiler_summary'></div><input id='memoryprofiler_clear_alloc_stats' type='button' value='Clear alloc stats' ></input><br />Sort allocations by:<select id='memoryProfilerSort'><option value='bytes'>Bytes</option><option value='count'>Count</option><option value='fixed'>Fixed</option></select><div id='memoryprofiler_ptrs'></div>";
     }
     var populateHtmlBody = function() {