fix(shared): support custom .toString() in text interpolation again

Vue 2 did this, but this functionality was quietly removed
from Vue 3 as a side effect of 3c60d40.

Closes vuejs#3944.

packages/shared/__tests__/toDisplayString.spec.ts

@@ -31,10 +31,22 @@ describe('toDisplayString', () => {
       })
     ).toBe(JSON.stringify({ n: 1, np: 2 }, null, 2))
   })
+
+  test('objects with custom toString', () => {
+    class TestClass {
+      toString() {
+        return 'foo'
+      }
+    }
+    const instance = new TestClass()
+    expect(toDisplayString(instance)).toBe('foo')
+    const obj = { toString: () => 'bar' }
+    expect(toDisplayString(obj)).toBe('bar')
+  })
 
   test('native objects', () => {
     const div = document.createElement('div')
-    expect(toDisplayString(div)).toBe(`"[object HTMLDivElement]"`)
+    expect(toDisplayString(div)).toBe('[object HTMLDivElement]')
     expect(toDisplayString({ div })).toMatchInlineSnapshot(`
       "{
         \\"div\\": \\"[object HTMLDivElement]\\"

packages/shared/src/toDisplayString.ts

@@ -1,4 +1,11 @@
-import { isArray, isMap, isObject, isPlainObject, isSet } from './index'
+import {
+  isArray,
+  isMap,
+  isObject,
+  isPlainObject,
+  isSet,
+  objectToString
+} from './index'
 
 /**
  * For converting {{ interpolation }} values to displayed strings.
@@ -7,7 +14,7 @@ import { isArray, isMap, isObject, isPlainObject, isSet } from './index'
 export const toDisplayString = (val: unknown): string => {
   return val == null
     ? ''
-    : isObject(val)
+    : isArray(val) || (isObject(val) && val.toString === objectToString)
     ? JSON.stringify(val, replacer, 2)
     : String(val)
 }