feat: pass vencord variable when compiling

this is another one of those standards-defining things like
`vc-requiredPlugins`, but i do genuinely think passing the client mod
that's in use here will be beneficial in the long run for usercss
authors since they can write one file that works across multiple.
hopefully other mods that implement usercss will adopt this 🤞

src/utils/themes/usercss/compiler.ts

@@ -75,7 +75,9 @@ export async function compileUsercss(fileName: string) {
         return null;
     }
 
-    const varsToPass = {};
+    const varsToPass = {
+        vencord: "true"
+    };
 
     for (const [k, v] of Object.entries(vars)) {
         varsToPass[k] = Settings.userCssVars[id]?.[k] ?? v.default;