Helpers overhaul

[bzm3r]

Per discussions, an overhaul of helpers that also with the following issues:

• https://github.com/StarArawn/bevy_ecs_tilemap/issues/30

• https://github.com/StarArawn/bevy_ecs_tilemap/issues/58 (hopefully, testing must confirm)

It provides the ground work for dealing with (but does not yet generate mouse events):

• https://github.com/StarArawn/bevy_ecs_tilemap/issues/40

What remains before the PR can move out of draft stage:

• [ ] docs need to be updated/cleaned up (there are a lot of docs around that no longer belong where they are, or need to be simplified)

• [x] update hex shader code to use new tile position to world position code for hexagons

• [ ] create an example that uses the new code, and confirms that world position -> tile position works correctly

An extension to this PR would create mouse events as requested in #40