Add global constants to distinguish current ENV

[2nthony]

Now idea is check package.json

If have vue then set __DVAN_ENV__ = 'vue', cannot set VUE_ENV because vue-ssr using it(If I still remember).

If have React then set __DVAN_ENV__ = 'react'

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Maybe this way is better: If have vue then set __DVAN_ENV__ to vue, react in otherwise.

[2nthony]

Actually use __DVAN_ENV__: string in Client-Side, also have __IS_VUE__: boolean

Since version: 2.1.5