Fix Issue 13212 - Windows line endings handled incorrectly by ddoc macros

https://issues.dlang.org/show_bug.cgi?id=13212

Handle `\r` as `\n` in macro definitions to prevent a trailing carriage return on a macro with an empty line following it.
Previously `FOO=BAR\r\n\r\n` would define `FOO` as `BAR\r\n`, when it should be just `BAR`.