Shell: rewrite the whole shell to use tokens instead of raw strings

tab completion is still running with raw strings and that has to be
fixed in the future.

userspace/programs/Shell/Execute.h

@@ -0,0 +1,53 @@
+#pragma once
+
+#include "Builtin.h"
+#include "CommandTypes.h"
+
+#include <BAN/NoCopyMove.h>
+
+class Execute
+{
+	BAN_NON_COPYABLE(Execute);
+	BAN_NON_MOVABLE(Execute);
+public:
+	Execute() = default;
+
+	BAN::ErrorOr<int> execute_command_sync(BAN::Span<const BAN::String> arguments, int fd_in, int fd_out);
+	BAN::ErrorOr<void> execute_command(const SingleCommand&, int fd_in, int fd_out, bool background, pid_t pgrp = 0);
+	BAN::ErrorOr<void> execute_command(const PipedCommand&);
+	BAN::ErrorOr<void> execute_command(const CommandTree&);
+
+	BAN::ErrorOr<void> source_script(BAN::StringView path);
+
+	int last_background_pid() const { return m_last_background_pid; }
+	int last_return_value() const { return m_last_return_value; }
+
+private:
+	struct InternalCommand
+	{
+		enum class Type
+		{
+			Builtin,
+			External,
+		};
+
+		BAN::Variant<Builtin::BuiltinCommand, BAN::String> command;
+		BAN::Span<const BAN::String> arguments;
+		int fd_in;
+		int fd_out;
+		bool background;
+		pid_t pgrp;
+	};
+
+	struct ExecuteResult
+	{
+		pid_t pid;
+		int exit_code;
+	};
+
+	BAN::ErrorOr<ExecuteResult> execute_command_no_wait(const InternalCommand& command);
+
+private:
+	int m_last_background_pid { 0 };
+	int m_last_return_value { 0 };
+};