Add UMD global declaration to index.d.ts

[RyanCavanaugh]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

Users using Vue from e.g. the CDN might be using the global .Vue without importing the module.

See https://www.typescriptlang.org/docs/handbook/modules.html for details on the syntax

[FranklinWhale]

@yyx990803 This PR is missing in the 2.5.17 release. Is that intended?

[posva]

yes, the latest release only included a security fix

[FranklinWhale]

@posva Thanks the info. I thought that 2.5.17 included all changes v2.5.17-beta.0 and was a normal release.

@RyanCavanaugh I installed this type definition using npm install github:vuejs/vue. However, the following script is marked as an error by the TypeScript compiler:

const vm = new Vue({
	el: "#test",
	data: "test"
});
// ERROR [ts] Cannot use 'new' with an expression whose type lacks a call or construct signature.

Since the type definition of Vue is specified in package.json, I have set the value of the moduleResolution compiler option to node for the compiler to resolve it. The full content of tsconfig.json is as follows:

{
	"compilerOptions": {
		"target": "es2018",
		"strict": true,
		"noImplicitReturns": true,
		"moduleResolution": "node",
		"outDir": "dist"
	}
}

There is also an index.d.ts file in the project to reference the UMD globally:

/// <reference types="vue"/>

Is there anything missing in my scripts?