made @root package available to userland code

[emekoi]

this closes #2189 and introduces the package @root to all userland code.

[daurnimator]

minor nit: commit message has a typo ("infite vs infinite")

[hryx]

Why is this @root when the other special imports are std and builtin (without the @ sign)? Feels like they should be consistent.

[emekoi]

i just used @root because that what was already being used. that also is consistent with @build referring to the build.zig file. if that were to be changed I think that would be up to @andrewrk.