Implement new modal designs

They aren't yet used anywhere, but this will let us move on to implementing specific modal interactions from the new designs.

I made the design decision of making this new Modal component always be controlled by an explicit open state, which was inspired by some work I did with Jetpack Compose recently, where I saw that this makes state management and the behavior of components so much more obvious.

src/Glass.tsx

@@ -0,0 +1,52 @@
+/*
+Copyright 2023 New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import {
+  ComponentPropsWithoutRef,
+  ReactNode,
+  forwardRef,
+  Children,
+} from "react";
+import classNames from "classnames";
+
+import styles from "./Glass.module.css";
+
+interface Props extends ComponentPropsWithoutRef<"div"> {
+  children: ReactNode;
+  className?: string;
+  /**
+   * Increases the blur effect.
+   * @default false
+   */
+  frosted?: boolean;
+}
+
+/**
+ * Adds a border of glass around a child component.
+ */
+export const Glass = forwardRef<HTMLDivElement, Props>(
+  ({ frosted = false, children, className, ...rest }, ref) => (
+    <div
+      ref={ref}
+      className={classNames(className, styles.glass, {
+        [styles.frosted]: frosted,
+      })}
+      {...rest}
+    >
+      {Children.only(children)}
+    </div>
+  )
+);