Add env stdlib

[antfu]

This resolves #449

Since "different lib implementations for different target languages" only works for stdlib but not user-defined libs. So this simple stdlib can help users to detect which language is currently using.

吾嘗觀「「天地經」」之書。方悟「境」之義。

若「境」等於「「犀」」者。
	批曰。「「Javascript也」」
或若「境」等於「「蟒」」者。
	批曰。「「Python也」」
或若「境」等於「「鑽」」者。
	批曰。「「Ruby也」」
若非
	批曰。「「不知所以也」」
也。

Naming are placeholders, feel free to adjust them :)

[LingDong-]

Great! Thanks for the addition. Names are alright, we can change them if we ever think of something better later.