Introduce distinction between DEV and PROD builds

[mweststrate]

Can we lift on what most people do with React as well, e.g. use process.env.NODE_ENV substitutions and conditions?

Things that could be stripped on PROD builds:

• Detailed error messages (see Save 8kb by replaceing error messages with error codes #653 )
• Many internal variant checks
• Don't show deprecation messages
• Give additional warnings & hings in dev builds, see e.g. Dealing with the MobX array != real array footgun #806

[tanzim]

@mweststrate I can take this on if no one has shown interest yet. Minor caveat is I haven't worked with TS, so bootstrapping time might be a little longer for me :)

[mweststrate]

Go for it! If you need process, you probably need put declare var process:any in somewhere, to signal that that is a globally available variable. Besides that typescript should hopefully not be too much in your way :)

[hccampos]

Cool stuff. Although MobX is not that big, it is always nice to get it a bit smaller. :)

[mweststrate]

This issue will be fixed in MobX 4