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/SectionNode.h

@@ -37,6 +37,8 @@ public:
     virtual bool is_open() const override { return m_open; }
     void set_open(bool open);
 
+    static ErrorOr<NonnullRefPtr<SectionNode>> try_create_from_number(StringView section_number);
+
 protected:
     String m_section;
     String m_name;