keys: Define key-check for primary directional keys.

This commit adds a function that checks if a key is one of the
directional keys(i.e. right, left, up or down). This will allow
us to have a more general and concise way of checking for direction
keys during keypress events.

zulipterminal/config/keys.py

@@ -342,6 +342,10 @@ class KeyBinding(TypedDict, total=False):
     ('msg_compose', 'Composing a Message'),
 ])
 
+DIRECTIONAL_COMMANDS = [
+    'GO_UP', 'GO_RIGHT', 'GO_LEFT', 'GO_DOWN'
+]
+
 
 class InvalidCommand(Exception):
     pass
@@ -381,3 +385,12 @@ def commands_for_random_tips() -> List[KeyBinding]:
     """
     return [key_binding for key_binding in KEY_BINDINGS.values()
             if not key_binding.get('excluded_from_random_tips', False)]
+
+
+def is_primary_direction_key(key: str) -> bool:
+    """
+    Returns True if the key is one of the directional keys
+    else False.
+    """
+    return key in [primary_key_for_command(command)
+                   for command in DIRECTIONAL_COMMANDS]