midiInstrument: support

[apple502j]

Resolves

#861
Projects with old 1.4 "midiInstrument:" block fail to open

Proposed Changes

Supporting midiInstrument: as Scratch 2.0 supporting.
src/serialization/sb2_specmap.js added

'midiInstrument:':{
+        opcode: 'music.setInstrument',
+        argMap: [
+            {
+                type: 'input',
+                inputOp: 'math_number',
+                inputName: 'INSTRUMENT'
+            }
+        ]
+    },

Reason for Changes

Old one cannot read 1.4 projects with this block.

Test Coverage

Read 2.0 project uses it and was uploaded by 1.4 like
https://llk.github.io/scratch-gui/develop/#138256836

[towerofnix]

This definitely fixes #861 in the sense that those projects will now actually load, but I don't think it matches compatibility with 2.0, which uses a mapping of 1.4 MIDI instrument IDs to 2.0+ "Scratch instrument" IDs. (See discussion in scratchfoundation/scratch-audio#13.)

[thisandagain]

@ericrosenbaum Can you please review / provide feedback?

[thisandagain]

@ericrosenbaum Bump