patch 8.2.0600: Vim9: cannot read or write w:, t: and b: variables

Problem:    Vim9: cannot read or write w:, t: and b: variables.
Solution:   Implement load and store for w:, t: and b: variables.
            (closes #5950)
diff --git a/src/vim9.h b/src/vim9.h
index 57cbfea..d5d4fd9 100644
--- a/src/vim9.h
+++ b/src/vim9.h
@@ -20,6 +20,9 @@
     ISN_LOAD,	    // push local variable isn_arg.number
     ISN_LOADV,	    // push v: variable isn_arg.number
     ISN_LOADG,	    // push g: variable isn_arg.string
+    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_LOADS,	    // push s: variable isn_arg.loadstore
     ISN_LOADSCRIPT, // push script-local variable isn_arg.script.
     ISN_LOADOPT,    // push option isn_arg.string
@@ -29,6 +32,9 @@
     ISN_STORE,	    // pop into local variable isn_arg.number
     ISN_STOREV,	    // pop into v: variable isn_arg.number
     ISN_STOREG,	    // pop into global variable isn_arg.string
+    ISN_STOREB,	    // pop into buffer-local variable isn_arg.string
+    ISN_STOREW,	    // pop into window-local variable isn_arg.string
+    ISN_STORET,	    // pop into tab-local variable isn_arg.string
     ISN_STORES,	    // pop into script variable isn_arg.loadstore
     ISN_STORESCRIPT, // pop into script variable isn_arg.script
     ISN_STOREOPT,   // pop into option isn_arg.string