fix(core): fix selector functions may escape plugin context

packages/koishi-core/src/context.ts

@@ -57,16 +57,12 @@ export class Context {
   protected _router: Router
   protected _database: Database
 
-  public user = this.createSelector('userId')
-  public self = this.createSelector('selfId')
-  public group = this.createSelector('groupId')
-  public channel = this.createSelector('channelId')
-  public platform = this.createSelector('platform')
-
   protected constructor(public filter: Filter, public app?: App, private _plugin: Plugin = null) {}
 
   [inspect.custom]() {
-    return `Context {}`
+    const plugin = this._plugin
+    const name = !plugin ? 'root' : typeof plugin === 'object' && plugin.name || 'unknown'
+    return `Context <${name}>`
   }
 
   private createSelector<K extends keyof Session>(key: K) {
@@ -75,6 +71,26 @@ export class Context {
     return selector
   }
 
+  get user() {
+    return this.createSelector('userId')
+  }
+
+  get self() {
+    return this.createSelector('selfId')
+  }
+
+  get group() {
+    return this.createSelector('groupId')
+  }
+
+  get channel() {
+    return this.createSelector('channelId')
+  }
+
+  get platform() {
+    return this.createSelector('platform')
+  }
+
   get private() {
     return this.unselect('groupId').user
   }