patch 9.0.2076: Vim9: No support for type aliases

Problem:  Vim9: No support for type aliases
Solution: Implement :type command

A type definition is giving a name to a type specification.  This also known
type alias.

	:type ListOfStrings = list<string>

The type alias can be used wherever a built-in type can be used.  The type
alias name must start with an upper case character.

closes: #13407

Signed-off-by: Christian Brabandt <cb@256bit.org>
Signed-off-by: Yegappan Lakshmanan <yegappan@yahoo.com>
diff --git a/src/vim9execute.c b/src/vim9execute.c
index a6bf471..7901f6e 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -3807,6 +3807,13 @@
 	    case ISN_STORE:
 		--ectx->ec_stack.ga_len;
 		tv = STACK_TV_VAR(iptr->isn_arg.number);
+		if (STACK_TV_BOT(0)->v_type == VAR_TYPEALIAS)
+		{
+		    semsg(_(e_using_typealias_as_variable),
+				STACK_TV_BOT(0)->vval.v_typealias->ta_name);
+		    clear_tv(STACK_TV_BOT(0));
+		    goto on_error;
+		}
 		clear_tv(tv);
 		*tv = *STACK_TV_BOT(0);
 		break;
@@ -7517,6 +7524,7 @@
 	case VAR_INSTR:
 	case VAR_CLASS:
 	case VAR_OBJECT:
+	case VAR_TYPEALIAS:
 	    break;
     }
     return FALSE;