fix: css/global not handle the exports name

[ahabhgk]

According the definition of the css/global and css module type, css/global should handle the exports name, css shouldn't, but now is reversed, this PR aimed to fix that.

Besides that, this PR also does:

• the ident of @keyframe and @property should keep it as is in css/global
• exportsConvention is not needed for css module type, since :export is a css/global css/module feature, so remove it from css generator options

What kind of change does this PR introduce?

Did you add tests for your changes?

updated snapshot

Does this PR introduce a breaking change?

yes, exportsConvention is remove from css generator options

What needs to be documented once your changes are merged?

update css generator options

[webpack-bot]

For maintainers only:

• This needs to be documented (issue in webpack/webpack.js.org will be filed when merged)
• This needs to be backported to webpack 4 (issue will be created when merged)