Allow monitors for mouse down, mouse x, mouse y, days since 2000

scratch-vm part. Not strictly necessary, but is more consistent with existing monitors.
https://github.com/TurboWarp/scratch-gui/issues/634

src/blocks/scratch3_sensing.js

@@ -81,12 +81,24 @@ class Scratch3SensingBlocks {
             sensing_answer: {
                 getId: () => 'answer'
             },
+            sensing_mousedown: {
+                getId: () => 'mousedown'
+            },
+            sensing_mousex: {
+                getId: () => 'mousex'
+            },
+            sensing_mousey: {
+                getId: () => 'mousey'
+            },
             sensing_loudness: {
                 getId: () => 'loudness'
             },
             sensing_timer: {
                 getId: () => 'timer'
             },
+            sensing_dayssince2000: {
+                getId: () => 'dayssince2000'
+            },
             sensing_current: {
                 // This is different from the default toolbox xml id in order to support
                 // importing multiple monitors from the same opcode from sb2 files,