Image Field Creation Doc Updates

[BeksOmega]

Where

https://developers.google.com/blockly/guides/create-custom-blocks/fields/built-in-fields/image#creation

What

• Text
• Image or Gif
• Other

Old content

src	A string that points to a raster image file. If it is null or undefined an empty string will be used.

Suggested content

src	A string that points to a raster image file.

The raster image link should also open in a new tab (since it has the new tab icon) which it currently does not.

Additional context

#2701

The src parameter is now required, as an empty string (old default) is still invalid.