Shell: Add an option to autosave history every N ms

...and set it to 10 seconds by default.

Base/etc/shellrc

@@ -38,3 +38,5 @@ if [ "$(id -u)" = "0" ] {
 }
 
 export PROMPT="\\X\\u@\\h:\\w\\a\\e[$prompt_color;1m\\h\\e[0m:\\e[34;1m\\w\\e[0m \\p "
+
+export HISTORY_AUTOSAVE_TIME_MS=10000

Base/usr/share/man/man7/Shell-vars.md

@@ -33,6 +33,13 @@ Note: This variable is respected by every program using `Line::Editor`, e.g. [`j
 The value of this variable is used as the Shell's history file path, both for reading history at startup and writing history on exit.
 Its default value is `~/.history`.
 
+`HISTORY_AUTOSAVE_TIME_MS` (environment)
+
+Setting this variable to a value `t` other than zero (0) will make the shell save the history to the history file every `t` milliseconds.
+If `t` is not a non-negative integer, zero will be assumed.
+Note that multiple shell instances will not interfere with each other if they are to save to the same history file, instead, the entries will all be merged together chronologically.
+The default value for this option is set in `/etc/shellrc`.
+
 ## Visual
 
 1. Prompting

Userland/Shell/Shell.cpp

@@ -1844,6 +1844,8 @@ Shell::Shell(Line::Editor& editor, bool attempt_interactive)
 
         return EDITOR_INTERNAL_FUNCTION(finish)(editor);
     });
+
+    start_timer(3000);
 }
 
 Shell::~Shell()
@@ -2056,6 +2058,36 @@ Optional<int> Shell::resolve_job_spec(const String& str)
     return {};
 }
 
+void Shell::timer_event(Core::TimerEvent& event)
+{
+    event.accept();
+
+    if (m_is_subshell)
+        return;
+
+    StringView option = getenv("HISTORY_AUTOSAVE_TIME_MS");
+
+    auto time = option.to_uint();
+    if (!time.has_value() || time.value() == 0) {
+        m_history_autosave_time.clear();
+        stop_timer();
+        start_timer(3000);
+        return;
+    }
+
+    if (m_history_autosave_time != time) {
+        m_history_autosave_time = time.value();
+        stop_timer();
+        start_timer(m_history_autosave_time.value());
+    }
+
+    if (!m_history_autosave_time.has_value())
+        return;
+
+    if (m_editor)
+        m_editor->save_history(get_history_path());
+}
+
 void FileDescriptionCollector::collect()
 {
     for (auto fd : m_fds)

Userland/Shell/Shell.h

@@ -276,6 +276,8 @@ private:
     Shell();
     virtual ~Shell() override;
 
+    void timer_event(Core::TimerEvent&) override;
+
     // FIXME: Port to Core::Property
     void save_to(JsonObject&);
     void bring_cursor_to_beginning_of_a_line() const;
@@ -347,6 +349,8 @@ private:
     bool m_default_constructed { false };
 
     mutable bool m_last_continuation_state { false }; // false == not needed.
+
+    Optional<size_t> m_history_autosave_time;
 };
 
 [[maybe_unused]] static constexpr bool is_word_character(char c)