Shell: Allow completions to request further action from the shell

The shell now expects a JSON object of the form {"kind":<kind>,...} per
line in the standard output of the completion process, where 'kind' is
one of:
- "plain": Just a plain suggestion.
- "program": Prompts the shell to complete a program name starting with
  the given "name".
- "proxy": Prompts the shell to act as if a completion for "argv" was
  requested.
- "path": Prompts the shell to complete a path given the "base" and
  "part" (same as completing part in cwd=base).

Userland/Shell/Shell.h

@@ -12,6 +12,7 @@
 #include <AK/CircularQueue.h>
 #include <AK/HashMap.h>
 #include <AK/NonnullOwnPtrVector.h>
+#include <AK/StackInfo.h>
 #include <AK/String.h>
 #include <AK/StringBuilder.h>
 #include <AK/StringView.h>
@@ -223,6 +224,7 @@ public:
 
     void highlight(Line::Editor&) const;
     Vector<Line::CompletionSuggestion> complete();
+    Vector<Line::CompletionSuggestion> complete(StringView);
     Vector<Line::CompletionSuggestion> complete_program_name(StringView, size_t offset, EscapeMode = EscapeMode::Bareword);
     Vector<Line::CompletionSuggestion> complete_variable(StringView, size_t offset);
     Vector<Line::CompletionSuggestion> complete_user(StringView, size_t offset);
@@ -419,6 +421,8 @@ private:
     mutable bool m_last_continuation_state { false }; // false == not needed.
 
     Optional<size_t> m_history_autosave_time;
+
+    StackInfo m_completion_stack_info;
 };
 
 [[maybe_unused]] static constexpr bool is_word_character(char c)