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.module.css

@@ -0,0 +1,31 @@
+/*
+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.
+*/
+
+.glass {
+  border-radius: 36px;
+  padding: 11px;
+  border: 1px solid var(--cpd-color-alpha-gray-400);
+  background: var(--cpd-color-alpha-gray-400);
+  backdrop-filter: blur(10px);
+}
+
+.glass > * {
+  border-radius: 24px;
+}
+
+.glass.frosted {
+  backdrop-filter: blur(20px);
+}