Feat: add webp CLI to devcontainer

[angela-tran]

Part of #2490

This is an optional PR. If we don't want this in the devcontainer, we can just close this.

This PR installs CLI tools from Google, including cwebp, to create and work with WebP images, giving us a consistent way to create WebP images rather than each of us doing it our own way, e.g. using online tools like https://squoosh.app/ (from https://github.com/GoogleChromeLabs/squoosh) or using ImageMin, etc.

I couldn't find a way to just install cwebp, which is what I think we'd mainly use; the full webp package contains cwebp, so just install the whole thing.

Trying it locally

Rebuild the devcontainer, and then in a terminal, run cwebp to see the help screen.

calitp@415aa65007bd:/calitp/app$ cwebp
Usage:

   cwebp [options] -q quality input.png -o output.webp

where quality is between 0 (poor) to 100 (very good).
Typical value is around 80.

Try -longhelp for an exhaustive list of advanced options.

[github-actions]

Coverage report

This PR does not seem to contain any modification to coverable code.

[thekaveman]

Eh, why not?