De-duplicate web build code

[TimJentzsch]

Objective

Closes #229.

The bevy run web command cannot just wrap cargo run, as we don't want to run a native application, but run the code in the user's browser.
Hence, we have to manually build with cargo build instead.

This causes quite a lot of code (also including calling wasm-bindgen) to be duplicated between bevy build web and bevy run web.

To ensure consistency and aid development, the code should be shared where possible.
This will enable us to build more complex features like #226.

Solution

• Extract reusable part of bevy build web into dedicated function.
• Add a conversion from run args to build args.
• Call the bevy build web logic in bevy run web as well.

The functionality should be equivalent.

In the future, we might refine how exactly the shared build function is called.
For example, I assume that we will pass the binary targets we want to build from the outside instead of determining it in the function itself, that will enable us to implement changes like #200 and #230.

[DaAlbrecht]

Nice!, the duplicated logic was annoying me too ^^.

I tested bevy run web and bevy build weg both still work.