ts: Migrate popover_menus to typescript.

This commit migrates `popover_menus` module to TypeScript. Also adds
a placeholder types declaration file for `css_variables` module.

web/src/css_variables.d.ts

@@ -0,0 +1,33 @@
+/* This is a placeholder typescript declaration file for `css_variables` module.
+   We can't convert the `css_variables` module to typescript yet because converting
+   it causes Webpack to trigger type checking with the TypeScript compiler, which is very expensive.
+
+   TS-migration of this module was reverted in this PR: https://github.com/zulip/zulip/pull/24985.
+*/
+
+declare const css_variables: {
+    media_breakpoints: {
+        xs_min: string;
+        sm_min: string;
+        md_min: string;
+        mc_min: string;
+        lg_min: string;
+        xl_min: string;
+        ml_min: string;
+        mm_min: string;
+        ms_min: string;
+    };
+    media_breakpoints_num: {
+        xs: number;
+        sm: number;
+        md: number;
+        mc: number;
+        lg: number;
+        xl: number;
+        ml: number;
+        mm: number;
+        ms: number;
+    };
+};
+
+export = css_variables;