Add tokenizer unicode for <->, and ->

The WL values doesn't seem to be shown on terminals.
We'll accept them, but also accept the real Unicode versions too.

This is coordinated with mathicsscript which adds escape sequences
for the Unicode arrows

The WL arrows you'd get from a cut and paste of WL examples.

Author: rocky

mathics/core/parser/tokeniser.py

@@ -96,9 +96,13 @@
     ("Greater", r" \> "),
     ("Less", r" \< "),
     # https://reference.wolfram.com/language/ref/character/UndirectedEdge.html
+    # The official Unicode value is below that.
     ("UndirectedEdge", r" -> | \uf3d4"),
+    ("UndirectedEdge", r" -> | \u2194"),
     # https://reference.wolfram.com/language/ref/character/DirectedEdge.html
+    # The official Unicode value is below that.
     ("DirectedEdge", r" -> | \uf3d5"),
+    ("DirectedEdge", r" -> | \u2192"),
     ("Or", r" (\|\|) | \u2228 "),
     ("And", r" (\&\&) | \u2227 "),
     ("RepeatedNull", r" \.\.\. "),