Conditional Compilation not documented

[erikwatson]

There should be a list of the available conditional compilation flags we can use documented somewhere.

There used to be one here - https://www.openfl.org/archive/developer/documentation/conditional-compilation/
It's still linked to by the HaxeFlixel docs, but it's a dead link.

Edit: I am noticing now that there is some mention of them in the Lime docs instead, though I really think it needs to be easier to find from the OpenFl site.

[loudoweb]

I've started a spreadsheet but I can't finish it without help: https://docs.google.com/spreadsheets/d/1wIObN7nelqfcDc1o4mMhKDXfWgwFdPqsv0nO0pqLj-o/edit#gid=0