Various TypeScript improvements (#2309)

- Use Mithril.Attributes as base for ComponentAttrs, remove =any from class signature for Component
- Convert Alert to TypeScript, introduce AlertAttrs interface
- Convert AlertManagerState to TypeScript, add overload signatures for `show`, introduce AlertState interface for stored Alerts.
- Set ComponentAttrs as default T for Component
- Make attrs in AlertAttrs optional
- Add AlertIdentifier interface, simplify show type signature
- Remove mithril patch shim, as all patches onto m are now deprecated
- Use Mithril.Static for shim

framework/core/js/shims.d.ts

@@ -1,6 +1,5 @@
 // Mithril
-import * as Mithril from 'mithril';
-import Stream from 'mithril/stream';
+import Mithril from 'mithril';
 
 // Other third-party libs
 import * as _dayjs from 'dayjs';
@@ -9,13 +8,6 @@ import * as _$ from 'jquery';
 // Globals from flarum/core
 import Application from './src/common/Application';
 
-/**
- * Helpers that flarum/core patches into Mithril
- */
-interface m extends Mithril.Static {
-  prop: typeof Stream;
-}
-
 /**
  * Export Mithril typings globally.
  *
@@ -36,7 +28,7 @@ export as namespace Mithril;
  */
 declare global {
   const $: typeof _$;
-  const m: m;
+  const m: Mithril.Static;
   const dayjs: typeof _dayjs;
 }