Add an optional capture for /invite to invite regex

Author: ChrisLovering

pydis_core/utils/regex.py

@@ -12,6 +12,7 @@
         r"([.,]|dot)li|"                         # or .li
         r"([.,]|dot)io"                          # or .io
     r")"
+    r"((/|slash|\\)(invite))?"                   # / or \ or 'slash' invite
     r"(/|slash|\\)"                              # / or \ or 'slash'
     r"(?P<invite>\S+)",                          # the invite code itself
     flags=re.IGNORECASE