fix(chat): adoptRootStyles @import rules interaction

Since `@import` was officially dropped from the specfication for
constructed style sheets, skip trying to insert any such CSS rules
and error out, preventing adopting the rest of the rules inside the
chat message.

Author: rkaraivanov

src/components/chat/utils.ts

@@ -110,7 +110,6 @@ export function isImageAttachment(
   );
 }
 
-// REVIEW: Maybe put that behind a configuration flag as this is nasty.
 export function chatMessageAdoptPageStyles(
   message: IgcChatMessageComponent
 ): void {
@@ -120,6 +119,10 @@ export function chatMessageAdoptPageStyles(
     try {
       const constructed = new CSSStyleSheet();
       for (const rule of sheet.cssRules) {
+        // https://drafts.csswg.org/cssom/#dom-cssstylesheet-insertrule:~:text=If%20parsed%20rule%20is%20an%20%40import%20rule
+        if (rule.cssText.startsWith('@import')) {
+          continue;
+        }
         constructed.insertRule(rule.cssText);
       }
       sheets.push(constructed);