Allow the literal 0_ in the grammar

Decimal literals having exactly one zero and some underscores after it
are accepted by DMD, but currently not allowed by the grammar.
Previously they were allowed by the grammar as octal literals, but they
have been removed.

Author: tim-dlang

spec/lex.dd

@@ -638,10 +638,14 @@ $(GNAME IntegerSuffix):
     $(B UL)
 
 $(GNAME DecimalInteger):
-    $(B 0)
+    $(B 0) $(GLINK Underscores)$(OPT)
     $(GLINK NonZeroDigit)
     $(GLINK NonZeroDigit) $(GLINK DecimalDigitsUS)
 
+$(GNAME Underscores):
+    $(B _)
+    $(GLINK Underscores) $(B _)
+
 $(GNAME BinaryInteger):
     $(GLINK BinPrefix) $(GLINK BinaryDigitsNoSingleUS)