Fixathon: Web app dependency updates part 2 (Sass) (#29037)

* Update sass-loader@16.0.2

* Update sass part 1 (actually update it)

* Update scss files to remove at-charset and use at-use instead of at-import

at-charset hasn't been needed since we switched to dart-sass, and
at-import is now deprecated. Most of this was done using sass-migrator.

We still use at-import for files imported from node_modules, but sass
doesn't seem to complain about that

* Inline at-extend rule that was causing an error

* Remove or inline at-use rules in components package

webapp/channels/build/emoji/make_emojis.mjs

@@ -433,10 +433,7 @@ for (const key of emojiFilePositions.keys()) {
     cssEmojis.push(`.emoji-${key} { background-position: ${emojiFilePositions.get(key)} }`);
 }
 
-const cssRules = `
-@charset "UTF-8";
-
-.emojisprite-preview {
+const cssRules = `.emojisprite-preview {
     width: ${EMOJI_SIZE_PADDED}px;
     max-width: none;
     height: ${EMOJI_SIZE_PADDED}px;