frontend: Add dropdown for timezone selection

crates/frontend/js-components/lib/timezones.ts

@@ -0,0 +1,12 @@
+let timezonesPromise = null
+export async function getTimezones() {
+  timezonesPromise ||= new Promise(async (resolve, reject) => {
+    try {
+      let response = await fetch('/frontend/_timezones.json')
+      resolve(await response.json())
+    } catch (e) {
+      reject(e)
+    }
+  })
+  return await timezonesPromise
+}