add a current session history menu (#378)

* add a current session history menu

* Update current_session_history_menu.nu

* Delete non_default_vars_menu.nu

custom-menus/README.md

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+# Custom menus folder 
+
+In this folder you can find custom menus for nushell. 
+
+To make them work:
+1. insert the code of chosen menu into the `menus` section of the `config.nu` file. 
+2. Check, that the name of the menu is uniqe
+3. Add a shortcut to call for the needed menu in the `keybindings` section of the `config.nu` file
+

custom-menus/current_session_history_menu.nu

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+# The part below should be pasted inside the 'menus' list of the 'config.nu' file 
+
+	{
+      # session menu
+      name: current_session_history_menu
+      only_buffer_difference: false
+      marker: "# "
+      type: {
+        layout: list
+        page_size: 10
+    }
+    style: {
+      text: green
+      selected_text: green_reverse
+      description_text: yellow
+    }
+    source: { |buffer, position|
+
+      history -l 
+      | where session_id == (history session)
+      | select command
+      | where command =~ $buffer
+      | each { |it| {value: $it.command } }
+      | reverse
+      | uniq
+    }
+
+# The part below should be pasted into the 'keybindgs' list of the 'config.nu' file
+
+    {
+      name: "current_session_history_menu"
+      modifier: alt
+      keycode: char_r
+      mode: emacs
+      event: { send: menu name: current_session_history_menu }
+    }