Add wasm/bevy_webgl2 support

[rparrett]

• Removes default features from bevy dependency, a lot of which we don't actually need at all
• Adds back the bevy features we actually do need for our default features
• Adds a "web" feature to bevy_tiled
  • Omits the bevy/bevy_wgpu feature, which does not work with wasm
  • Omits bevy/png feature because it's only used for our examples
• Adds a separate set of shaders that work with webgl2 and uses them if the "web" feature is selected

[dmtaub]

This looks fine to me, it seems like the changes from your branch that I was already using.

I could use some more context or others to test it so I know it doesn't break anything for others :)

Also, sorry, I was working on the readme when you submitted this, so you might want to merge the most recent branch

[rparrett]

I am learning new things about vscode all the time. It has an auto-mangle-markdown feature too! I'll fix that up.

I'm assuming you mean context regarding the feature-related changes, because it seems pretty easy to verify that the shaders continue to work properly.

The bevy_tiled was depending on bevy prior to this PR is actually not a good situation. Because features are additive, it means that anyone who is using default-features=false in their projects gets a face full of bevy's default features if they add bevy_tiled as a dependency.

So bevy plugins should include only the bevy features that are required.

https://github.com/bevyengine/bevy/blob/master/docs/plugins_guidelines.md#bevy-features

Let me know if there's something more specific that I can try to clarify. But the cargo feature resolver is a bit mysterious to me still, so we may have to learn about it together.

[rparrett]

Rebased