Understand Escape as another name for Esc

This allows us to remove special-case handling of Esc in
forward_modified_utf8(), which was always sending "<Esc>" to nvim even
when there were modifiers present.

Closes #12584

src/nvim/keymap.c

@@ -156,6 +156,7 @@ static const struct key_name_entry {
   { K_BS,              "BS" },
   { K_BS,              "BackSpace" },   // Alternative name
   { ESC,               "Esc" },
+  { ESC,               "Escape" },      // Alternative name
   { CSI,               "CSI" },
   { K_CSI,             "xCSI" },
   { '|',               "Bar" },