Support conditions for build type

[mrexodia]

For multi-config generators there is an annoying thing that you cannot easily make debug/release-only flags. The idea would be:

[target.foo]
type = "executable"
sources = ["..."]
debug.compile-options = ["/DEBUGME"]

This would emit the correct generator expression.

An open question is still how to handle conditional debug flags. Perhaps something like:

[target.foo]
debug = { condition.compile-options = ["/DEBUGME"] }