Some integration with Type Checking.

Now the UI highlights commands and numbers as you move the mouse, numbers
are blue, commands that type-check are green, commands that fail to
type-check are orange and will not be interpreted, and if there is no
stack effect information available for a command it is grey but you can
still attempt to execute it.

You can still evaluate whole expressions by selceting them and
right-inter-clicking before you release the left button, or by putting
the cursor on a line and typing ctrl-enter, which will run the whole
line.  These expressions are NOT (yet) type-checked.

joy/gui/mousebindings.py

@@ -65,6 +65,7 @@ class MouseBindingsMixin:
     self.bind("<ButtonRelease-3>", self.B3r)
 
     self.bind("<Any-Leave>", self.leave)
+    self.bind("<Motion>", self.scan_command)
 
   def B1d(self, event):
     '''button one pressed'''
@@ -167,6 +168,9 @@ class MouseBindingsMixin:
 
     return "break"
 
+  def scan_command(self, event):
+    self.update_command_word(event)
+
   def B1r(self, event):
     '''button one released'''
     self.B1_DOWN = False