chore: Improve npm package specification

[anncwb]

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:

As the esm and npm exports fields are gradually standardized, more and more tools will implement specific functions, such as importmap, according to these specifications.

jspm is a tool that automatically generates importmap. It mainly generates importmap source map through the exports field of package.json.

https://ga.jspm.io/npm:vue@2.6.14/dist/vue.runtime.esm.js This is an example of it and we can access it. But if we need to use it in the browser, we need to visit https://ga.jspm.io/npm:vue@2.6.14/dist/vue.esm.browser.js.

This is currently not possible, so we need to configure { "./dist/*": "./dist/*" } essed in jspm