Add a Gamepad Viewer tool to examples (bevyengine#6074)

# Objective

Give folks an easy way to test their gamepad with bevy.

~~This is a lot of very boring code for an example. Maybe it belongs in the "tools" directory?~~

## Solution

https://user-images.githubusercontent.com/200550/191884342-ace213c0-b423-449a-9295-530cbceaa19e.mp4

## Notes

This has brought to light (to me, anyway) some fairly major issues with gamepads on the web. See:

[WASM mappings (gilrs issue 107)](https://gitlab.com/gilrs-project/gilrs/-/issues/107)
[Inaccurate value for trigger button of Xbox gamepad with WASM (gilrs issue 121)](https://gitlab.com/gilrs-project/gilrs/-/issues/121)

Cargo.toml

@@ -1340,6 +1340,16 @@ description = "A simple way to view glTF models with Bevy. Just run `cargo run -
 category = "Tools"
 wasm = true
 
+[[example]]
+name = "gamepad_viewer"
+path = "examples/tools/gamepad_viewer.rs"
+
+[package.metadata.example.gamepad_viewer]
+name = "Gamepad Viewer"
+description = "Shows a visualization of gamepad buttons, sticks, and triggers"
+category = "Tools"
+wasm = false
+
 # Transforms
 [[example]]
 name = "global_vs_local_translation"

examples/README.md

@@ -291,6 +291,7 @@ Example | Description
 
 Example | Description
 --- | ---
+[Gamepad Viewer](../examples/tools/gamepad_viewer.rs) | Shows a visualization of gamepad buttons, sticks, and triggers
 [Scene Viewer](../examples/tools/scene_viewer.rs) | A simple way to view glTF models with Bevy. Just run `cargo run --release --example scene_viewer /path/to/model.gltf#Scene0`, replacing the path as appropriate. With no arguments it will load the FieldHelmet glTF model from the repository assets subdirectory
 
 ## Transforms