Add gui/harvest script

docs/gui/harvest.rst

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+gui/harvest
+============
+
+.. dfhack-tool::
+    :summary: Instantly harvest plants, shrubs, and crops in a selected area.
+    :tags: fort armok plants
+
+Opens a GUI window that lets you box-select an area of the map and
+instantly harvest all shrubs, mature farm crops, and fallen fruit or
+plant items within it.
+
+Harvested goods are automatically placed into the nearest available
+barrel or bag, or dropped on the ground if no container is free. You
+can also double-click a specific container to use it.
+
+Yield quantities are based on the best grower and herbalist skill
+levels in your fortress, or you can toggle forced maximum yields.
+
+Usage
+-----
+
+::
+
+    gui/harvest
+
+Click and drag on the map to select harvestable plants. Double-click
+on the ground (or on a container) to execute the harvest.
+
+In-window controls
+------------------
+
+``Ctrl-a``
+    Select all harvestable tiles on the current z-level.
+``Ctrl-c``
+    Clear the current selection.
+``Ctrl-m``
+    Toggle between simulating your best dwarf's skill level and
+    forcing maximum yields.
+``Ctrl-s``
+    Toggle whether saplings are included in the harvest.