Add support for custom opcodes

[tech-ticks]

settings.json example:

{
  "explorerscript.customOperations": [{
    "id": 1000,
    "name": "CheckInputStatus",
    "description": "Writes the currently pressed/held buttons into $EVENT_LOCAL",
    "args": [{
      "name": "mode",
      "type": "Number",
      "description": "0: pressed, 1: held"
    }]
  }]
}