feat: add browser ESM 'minified build' using babel-minify

[ghost]

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:

[posva]

Did you see #7110?

[ghost]

@posva Yes. But @yyx990803 did not provide minified build. This pr is against branch 'esm-browser'. The minified ESM fulfills @simonhaenisch's request in #7110

[yyx990803]

Thanks for the PR, however I think we should just use terser which can work on both ES5 and ES2015 builds. I'll improve #7110 to include a minified build.