Switch to wasm-opt cli (#303)

# Objective

Currently, we use `wasm-opt` as a library to optimize the wasm binaries.
This makes our compile times noticeably longer. To reduce compile and
therefore CI time we switch to use the `wasm-opt` CLI variant.

# Solution

1. Move the `wasm-opt` module to `external_cli`
2. Remove `wasm-opt` from the dependencies
3. Add `wasm-opt`to the `ensure_web_setup` function that installs the
latest available version if no version is present and the user confirms.

# Testing

No wasm opt
```sh
❯ cargo install --path .
❯ bevy run -r web -v
DEBUG Running: `rustup target list`
DEBUG Running: `wasm-bindgen --version`
 INFO Compiling to WebAssembly...
DEBUG Running: `cargo build --config profile.web.inherits="dev" --config profile.web-release.inherits="release" --config profile.web-release.strip="debuginfo" --config profile.web-release.opt-level="s" --bin cplx --profile web-release --target wasm32-unknown-unknown`
    Finished `web-release` profile [optimized] target(s) in 0.11s
 INFO Bundling JavaScript bindings...
DEBUG Running: `wasm-bindgen --no-typescript --out-name cplx --out-dir /bevy_complex_repo/app/target/wasm32-unknown-unknown/web-release --target web /bevy_complex_repo/app/target/wasm32-unknown-unknown/web-release/cplx.wasm`
 INFO No custom `web` folder found, using defaults.
 INFO Open your app at <http://localhost:4000>!
```
With `wasm-opt`

```
❯ cargo install --path . --features wasm-opt
❯ bevy run -r web -v
DEBUG Running: `rustup target list`
DEBUG Running: `wasm-bindgen --version`
DEBUG Running: `wasm-opt --version`
 INFO Compiling to WebAssembly...
DEBUG Running: `cargo build --config profile.web.inherits="dev" --config profile.web-release.inherits="release" --config profile.web-release.opt-level="s" --config profile.web-release.strip="debuginfo" --bin cplx --profile web-release --target wasm32-unknown-unknown`
    Finished `web-release` profile [optimized] target(s) in 0.10s
 INFO Bundling JavaScript bindings...
DEBUG Running: `wasm-bindgen --no-typescript --out-name cplx --out-dir /bevy_complex_repo/app/target/wasm32-unknown-unknown/web-release --target web /bevy_complex_repo/app/target/wasm32-unknown-unknown/web-release/cplx.wasm`
 INFO Optimizing with wasm-opt...
DEBUG Running: `wasm-opt --strip-debug -Oz -o /bevy_complex_repo/app/target/wasm32-unknown-unknown/web-release/cplx_bg.wasm /bevy_complex_repo/app/target/wasm32-unknown-unknown/web-release/cplx_bg.wasm`
 INFO Finished in 18.36s. Size reduced by 48%.
 INFO No custom `web` folder found, using defaults.
 INFO Open your app at <http://localhost:4000>!
```

Closes #289

Author: DaAlbrecht