fix(types): fix global namespace declaration for UMD bundle

[ktsn]

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:

In the current declaration, index.d.ts has export as namespace Vue which exposes all exported types in index.d.ts in global namespace. So the user can access them by just writing like Vue.ComponentOptions without import statement.

But this declaration is actually not correct because the Vue constructor type is declared under default which is correct in ESModule bundle but UMD bundle.

new Vue({}) // This provides compilation error in current type.

new Vue.default({}) // No compilation error.

To declare Vue constructor as just Vue in global namespace, we need to write export as namespace Vue with a common js export. So I've add umd.d.ts and do that in that file.

[yyx990803]

This should not affect existing usage with ES modules right?

[ktsn]

Right, it should not affect. The entry point (index.d.ts) exports are the same and if a file is ES module, global Vue is not available.