Add command to create a directory in a new category "files"

Use the alt + (left click) is tree view to create a directory and
shift + (middle click) to remove a project directory.

data/core/commands/files.lua

@@ -0,0 +1,13 @@
+local core = require "core"
+local command = require "core.command"
+
+command.add(nil, {
+  ["files:create-directory"] = function()
+    core.command_view:enter("New directory name", function(text)
+      local success, err = system.mkdir(text)
+      if not success then
+        core.error("cannot create directory %q: %s", text, err)
+      end
+    end)
+  end,
+})