Add --bundle option to web commands

[TimJentzsch]

Objective

Closes #68.

Adds the --bundle argument to the bevy build web and bevy run web commands.
This will pack all files needed for the web into a single folder.
The location of the bundle will be target/bevy_web/{profile}/{binary}.

This makes it a lot easier to deploy the app e.g. on itch.io or a generic web server.
It also unblocks the switch from trunk to the Bevy CLI for the Bevy 2D template: TheBevyFlock/bevy_new_2d#312.

Solution

• Add the --bundle option to the build and run web commands.
• Create a new WebBundle enum, which can represent a linked or packed bundle:
  • A linked bundle is what we use by default, which is optimized for dev. It keeps e.g. the asset folder and WASM artifacts in their place to avoid duplication and copy operations. When running the app, the local web server will "link" together all the files and folders needed to run the app.
  • A packed bundle is a folder that contains all the necessary web artifacts. They will be copied in the directory. This option is most useful to deploy the web app.

[BD103]

Tested and it works! (Even on Bevy 0.11, which was the only project I have available at the time.)

Once you address the non-nits, I'll approve and merge. :)

[TimJentzsch]

@BD103 I think I addressed all comments, let me know if you notice anything else!

[BD103]

Looks good, thank you!