Is there a system-wide way to configure the default newline character? If so, is there a way to query what this character is?
No, there is not any way to configure the newline character system wide.
Modern macOS applications typically use
\n (Unicode character U+000A) as the line separator.
Applications developed for Classic Mac OS (i.e, Mac OS 9 and earlier) often used
\r (Unicode U+000D) as a line separator, but this usage is extremely rare nowadays.
A good text editor will let you choose your line break: