Add cwebp task for converting images to WebP format

[pbearne]

Implemented 'cwebp' Grunt task to convert images to WebP format. Updated 'precommit:image' task to include 'cwebp:dynamic' ensuring WebP conversion during pre-commit.

Trac ticket: https://core.trac.wordpress.org/ticket/62188

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[desrosj]

I agree with the image quality likely being far too low. It seems Core uses 86 as the default for WebP. I think matching that is a good start.

Were there specific reasons for choosing grunt-cwebp over some of the other available options, such as Ayesh/grunt-webp? The former has not been updated in 4 years, but the latter was updated 2 years ago and @Ayesh is an active member of the community, so it may be easier to get any needed updates released.

There's also some additional options such as lossless, and deleteLarger in Ayesh/grunt-webp. I wonder if configuring these would be beneficial as well.