patch 9.0.1248: cannot export an interface

Problem:    Cannot export an interface. (Ernie Rael)
Solution:   Add the EX_EXPORT flag to :interface. (closes #11884)

Author: brammool

src/ex_cmds.h

@@ -758,7 +758,7 @@ EXCMD(CMD_intro,	"intro",	ex_intro,
 	EX_TRLBAR|EX_CMDWIN|EX_LOCK_OK,
 	ADDR_NONE),
 EXCMD(CMD_interface,	"interface",	ex_class,
-	EX_EXTRA|EX_TRLBAR|EX_CMDWIN|EX_LOCK_OK,
+	EX_EXTRA|EX_TRLBAR|EX_CMDWIN|EX_LOCK_OK|EX_EXPORT,
 	ADDR_NONE),
 EXCMD(CMD_isearch,	"isearch",	ex_findpat,
 	EX_BANG|EX_RANGE|EX_DFLALL|EX_WHOLEFOLD|EX_EXTRA|EX_CMDWIN|EX_LOCK_OK,

src/testdir/test_vim9_class.vim

@@ -822,6 +822,29 @@ def Test_interface_basics()
       endinterface
   END
   v9.CheckScriptFailure(lines, 'E1345: Not a valid command in an interface: return 5')
+
+  lines =<< trim END
+      vim9script
+      export interface EnterExit
+          def Enter(): void
+          def Exit(): void
+      endinterface
+  END
+  writefile(lines, 'XdefIntf.vim', 'D')
+
+  lines =<< trim END
+      vim9script
+      import './XdefIntf.vim' as defIntf
+      export def With(ee: defIntf.EnterExit, F: func)
+          ee.Enter()
+          try
+              F()
+          finally
+              ee.Exit()
+          endtry
+      enddef
+  END
+  v9.CheckScriptSuccess(lines)
 enddef
 
 def Test_class_implements_interface()

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1248,
 /**/
     1247,
 /**/