Send events more often for variable sets outside of builtin_set

When changing certain variables programmatically, ensure that events
are sent. Fixes #6653

tests/bind_mode_events.expect

@@ -0,0 +1,36 @@
+# vim: set filetype=expect:
+spawn $fish
+expect_prompt
+
+send "set -g fish_key_bindings fish_vi_key_bindings\r"
+expect_prompt
+
+send "echo ready to go\r"
+expect_prompt -re {\r\nready to go\r\n} {
+    puts "ready to go"
+}
+send "function add_change --on-variable fish_bind_mode ; set -g MODE_CHANGES \$MODE_CHANGES \$fish_bind_mode ; end\r"
+expect_prompt
+
+# normal mode
+send "\033"
+sleep 0.050
+
+# insert mode
+send "i"
+sleep 0.050
+
+# back to normal mode
+send "\033"
+sleep 0.050
+
+# insert mode again
+send "i"
+sleep 0.050
+
+send "echo mode changes: \$MODE_CHANGES\r"
+expect_prompt -re {\r\nmode changes: default insert default insert\r\n} {
+        puts "Correct mode changes"
+} unmatched {
+    puts "Incorrect mode changes"
+}

tests/bind_mode_events.expect.out

@@ -0,0 +1,2 @@
+ready to go
+Correct mode changes