Publish deprecation data

[shawnbot]

This is the first stab at publishing deprecation data, as described in #740. Here's what accessing it looks like:

JSON

const {versions} = require('@primer/css/dist/deprecations.json')
for (const [version, deprecations] of Object.entries(versions)) {
  console.log(`> ### ${version}`)
  for (const {selectors, message} of deprecations) {
    console.log(`  - \`${selectors.join('\`, \`')}\`: ${message}`)
  }
}

outputs:

13.0.0

• .btn-purple: Please don't make purple buttons.
• .text-pending: Please use the "text-yellow" class instead of "text-pending".
• .bg-pending: Please use the "bg-yellow-dark" class instead of "bg-pending".
• .columns, .column, .one-third, .two-thirds, .one-fourth, .one-half, .three-fourths, .one-fifth, .four-fifths: Please use grid classes.
• .centered: You can use the "mx-auto" class to center any element.

See the Markdown docs (and the published page) for usage instructions.

Closes #740

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[zeke]

Left a few little comments and suggestions, but overall this looks good!

Are you planning to add tests to validate the deprecations.json output?

[colebemis]

Looks good! 💯 Left a few comments. The only thing I feel strongly about is adding title front matter to the documentation page.

[colebemis]

🚀