patch 8.2.1250: Vim9: cannot use the g:, b:, t: and w: namespaces

Problem:    Vim9: cannot use the g:, b:, t: and w: namespaces.
Solution:   Add instructions to push a dict for the namespaces. (closes #6480)
diff --git a/src/vim9.h b/src/vim9.h
index 39d36f9..0d51c98 100644
--- a/src/vim9.h
+++ b/src/vim9.h
@@ -26,6 +26,10 @@
     ISN_LOADB,	    // push b: variable isn_arg.string
     ISN_LOADW,	    // push w: variable isn_arg.string
     ISN_LOADT,	    // push t: variable isn_arg.string
+    ISN_LOADGDICT,  // push g: dict
+    ISN_LOADBDICT,  // push b: dict
+    ISN_LOADWDICT,  // push w: dict
+    ISN_LOADTDICT,  // push t: dict
     ISN_LOADS,	    // push s: variable isn_arg.loadstore
     ISN_LOADOUTER,  // push variable from outer scope isn_arg.number
     ISN_LOADSCRIPT, // push script-local variable isn_arg.script.