add RGSS_VERSION (RGSS3)

Even though 3.0.1 is the current version, 3.0.0 is used, because mkxp doesn't
implement the changes like not raising a TypeError for 'Color.new == nil' yet.
3.0.1 (finally) returns false. Other changes don't affect mkxp (I think).

See <http://tkool.jp/support/download/rpgvxace/rpgvxace_update> (Japanese).

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The comparison change and maybe adding `RGSS_VERSION` for `rgssVer == 1` and `rgssVer == 2` requires some discussion.

I think mkxp should follow 3.0.1's behavior for `rgssVer == 3` as its usage is higher than 3.0.0, because the international VX Ace defaults to RGSS301.dll since its release.