feat: Change the icon loading mode

[zxuanhong]

appendIcon and createIcon icon loading mode is changed to html to facilitate css to change icon color

css change color

.djs-context-pad {
  > .group {
    .create-append-anything-append-element {
      fill: #1e80ff !important;
    }
  }
}
.djs-palette-entries {
  > .group {
    .create-append-anything-create-element {
      fill: #1e80ff !important;
    }
  }
}

Closes #5

[CLAassistant]

All committers have signed the CLA.

[smbea]

Thanks for your contribution @zxiaozhou ! Could you please lint your code to match our code style and CI does not fail? Make sure npm run lint runs smoothly.

[zxuanhong]

@smbea Have taken care of.