Shell: Add a 'setopt' builtin

This builtin sets (and unsets) boolean flags that alter the behaviour of
the shell.
The only flags added are
- inline_exec_keep_empty_segments: Keep empty segments in the result of
  splitting $(...) by $IFS
- verbose: Announce each command before executing it

It should be noted that the (rather extreme) verbosity of the names is
intentional, and will hopefully be alleviated by the next commit :^)

Shell/AST.h

@@ -238,15 +238,17 @@ public:
     virtual ~StringValue();
     virtual bool is_string() const override { return m_split.is_null(); }
     virtual bool is_list() const override { return !m_split.is_null(); }
-    StringValue(String string, String split_by = {})
+    StringValue(String string, String split_by = {}, bool keep_empty = false)
         : m_string(string)
         , m_split(move(split_by))
+        , m_keep_empty(keep_empty)
     {
     }
 
 private:
     String m_string;
     String m_split;
+    bool m_keep_empty { false };
 };
 
 class GlobValue final : public Value {