Implement set/change stretch blocks

src/blocks/scratch3_looks.js

@@ -255,6 +255,8 @@ class Scratch3LooksBlocks {
             looks_cleargraphiceffects: this.clearEffects,
             looks_changesizeby: this.changeSize,
             looks_setsizeto: this.setSize,
+            looks_changestretchby: this.doNothing,
+            looks_setstretchto: this.doNothing,
             looks_gotofrontback: this.goToFrontBack,
             looks_goforwardbackwardlayers: this.goForwardBackwardLayers,
             looks_size: this.getSize,

src/serialization/sb2_specmap.js

@@ -349,6 +349,26 @@ const specMap = {
             }
         ]
     },
+    'changeStretchBy:': {
+        opcode: 'looks_changestretchby',
+        argMap: [
+            {
+                type: 'input',
+                inputOp: 'math_number',
+                inputName: 'CHANGE'
+            }
+        ]
+    },
+    'setStretchTo:': {
+        opcode: 'looks_setstretchto',
+        argMap: [
+            {
+                type: 'input',
+                inputOp: 'math_number',
+                inputName: 'STRETCH'
+            }
+        ]
+    },
     'comeToFront': {
         opcode: 'looks_gotofrontback',
         argMap: [