bump to Koishi 4.8

dev/index.ts

-import { App } from 'koishi';
+import { Context } from 'koishi';
 import TargetPlugin from '../src';
 import ConsolePlugin from '@koishijs/plugin-console';
 import SandboxPlugin from '@koishijs/plugin-sandbox';
@@ -6,7 +6,7 @@ import DatabasePlugin from '@koishijs/plugin-database-memory';
 import * as Help from '@koishijs/plugin-help';
 import ExtrasInDev from './extras';
 
-const app = new App({
+const app = new Context({
   port: 14514,
   host: 'localhost',
   prefix: '.',

tests/sample.spec.ts

-import { App } from 'koishi';
+import { Context } from 'koishi';
 import TargetPlugin, { UseCache } from '../src';
 import { CacheKey, WithKey } from 'aragami';
 import { DefinePlugin, Provide, StarterPlugin } from 'koishi-thirdeye';
 
 describe('Test of plugin.', () => {
-  let app: App;
+  let app: Context;
 
   beforeEach(async () => {
-    app = new App();
+    app = new Context();
     app.plugin(TargetPlugin, {
       // redis: { uri: 'redis://localhost:6379' },
     });