Help+man+LibManual: Move argument handling to LibManual

This deduplicates argument handling logic from Help and man and makes it
more modular for future use cases. The argument handling works as
before: two arguments specify section and page (in this order), one
argument specifies either a page (the first section that it's found in
is used) or a path to a manpage markdown file.

Userland/Libraries/LibManual/CMakeLists.txt

@@ -1,4 +1,5 @@
 set(SOURCES
+    Node.cpp
     PageNode.cpp
     Path.cpp
     SectionNode.cpp