feat(esm build): build ES modules for browser

[zigomir]

fixes #2589
Reviving #2590 (I deleted fork already)

For build size references:

Old build output

Circular dependency: src/util/push-state.js -> src/util/scroll.js -> src/util/push-state.js
dist/vue-router.js 64.41kb
Circular dependency: src/util/push-state.js -> src/util/scroll.js -> src/util/push-state.js
dist/vue-router.min.js 23.74kb (gzipped: 8.36kb)
Circular dependency: src/util/push-state.js -> src/util/scroll.js -> src/util/push-state.js
dist/vue-router.common.js 64.76kb
Circular dependency: src/util/push-state.js -> src/util/scroll.js -> src/util/push-state.js
dist/vue-router.esm.js 64.74kb

vs

new build output

Circular dependency: src/util/push-state.js -> src/util/scroll.js -> src/util/push-state.js
dist/vue-router.js 64.41kb
Circular dependency: src/util/push-state.js -> src/util/scroll.js -> src/util/push-state.js
dist/vue-router.min.js 23.81kb (gzipped: 8.37kb)
Circular dependency: src/util/push-state.js -> src/util/scroll.js -> src/util/push-state.js
dist/vue-router.common.js 64.76kb
Circular dependency: src/util/push-state.js -> src/util/scroll.js -> src/util/push-state.js
dist/vue-router.esm.js 64.74kb
Circular dependency: src/util/push-state.js -> src/util/scroll.js -> src/util/push-state.js
dist/vue-router.esm.browser.js 60.45kb
Circular dependency: src/util/push-state.js -> src/util/scroll.js -> src/util/push-state.js
dist/vue-router.esm.browser.min.js 21.96kb (gzipped: 8.12kb)