Split up map icon textures into individual files

Like with experience orbs, Java Edition currently pulls sprites for map icons (banner markers, explorer map destination markers, etc.) from a sprite sheet, instead of from individual map icon files. To be more in line with other textures, this sprite sheet should be scrapped (like with paintings and particles in 1.14) and replaced with individual files for each different icon. This would give more liberty to resource pack artists, as this would likely allow for map icons to be animated if desired, as well as make managing textures easier in general.