fix(emoji): reserve square box for img.emoji to avoid CLS (#4685)

Twemoji SVGs are loaded from a CDN and the rendered <img> carries no
intrinsic dimensions. With only `height: 1.5em` set and `width: auto`,
the browser reserves zero width per emoji until each SVG downloads,
reflowing surrounding text on load and producing visible Cumulative
Layout Shift.

All Twemoji glyphs are square, so declaring `aspect-ratio: 1` lets the
browser derive the width immediately and reserve the correct box before
the SVG arrives.

Fixes #4681

Author: imorland

extensions/emoji/less/forum.less

@@ -1,5 +1,6 @@
 img.emoji {
   height: 1.5em;
+  aspect-ratio: 1; // reserve a square box while the SVG loads to avoid layout shift (CLS)
   margin: 0 .1em;
   vertical-align: -0.3em;
 }