Add link capability similar to compile

[genotrance]

compile => {.compile: "xyz.c".}

Need a similar capability for link.

link => {.link: "xyz.o".}

Typically code is compiled as part of the nim build but if there's something separate, like a resources file compiled using execute = "windres resource.rc resource.o" then need ability to link.

Can currently use pragma for near term but link will allow for wildcards, etc.