Image import identifier should match image file name.

[pixelzoom]

Related to #84.

The dev team recently agreed (in phetsims/chipper#1166) to make the import identifer match the file name (minus its suffix). And there will (soon?) be a lint rule that enforces this for image files. That convention is not being followed for image imports in this sim.

For example, in NumberPlayGameScreen.js:

import gameScreenIconImage from '../../images/game_screen_icon_png.js';

gameScreenIconImage does not match game_screen_icon_png.

Two alternatives:

// rename the image file
import gameScreenIcon_png from '../../images/gameScreenIcon_png.js';

// rename the import identifier
import game_screen_icon_png from '../../images/game_screen_icon_png.js';

[chrisklus]

Completed in phetsims/chipper#1177, closing.