Supports browser field in package.json

[shrinktofit]

There are still many npm packages using browser field in package.json to declare the browser edition entries of the package. Developers are eagerly looking forward to such support.

See also: https://forum.cocos.org/t/topic/119511/114

Formal description of browser field: https://github.com/defunctzombie/package-browser-field-spec

[PPpro]

Unit Test https://github.com/cocos/creator-programming/pull/16/files

[shrinktofit]

The following is some popular packages that use browser field:

• https://github.com/axios/axios (as one of npm top 10 packages)

@PPpro They're also good points to verify our implementation.

[PPpro]

we reajust the priority of this issue, for there are much more inserted task