Themes: add "description" key to i18n schema

[ramonjd]

A "description" key was added to theme.json in WordPress/gutenberg#45242

It was synced to Core in #4687

A corresponding entry to the theme.json i18n schema was not part of the change. This PR does that.

Here's the corresponding update to the wp-cli repo: wp-cli/i18n-command#408

See discussion: WordPress/gutenberg#45242 (comment)

Trac ticket: https://core.trac.wordpress.org/ticket/61543

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[ramonjd]

Committed in https://core.trac.wordpress.org/changeset/58632

Thanks @noisysocks!