chore: replace metalsmith-prism with highlight.js

[Trott]

Remove a metalsmith plugin that hasn't been updated recently. This has
several advantages:

• Removes some vulnerabilities the GitHub UI is complaining about.
• Eliminates an error message when building involving prism.
• Uses the same highlighting library as the docs, setting the stage for
  possible consolidation and/or alignment.

[richardlau]

• Eliminates an error message when building involving prism.

I'm assuming this fixes this error during the website build so 👍 :

stderr: Failed to load prism syntax: mjs
stderr: Error: Cannot find module 'prismjs/components/prism-mjs.js'
stderr: Require stack:
stderr: - /website/node_modules/metalsmith-prism/lib/index.js
stderr: - /website/build.js
stderr:     at Function.Module._resolveFilename (node:internal/modules/cjs/loader:933:15)
stderr:     at Function.Module._load (node:internal/modules/cjs/loader:778:27)
stderr:     at Module.require (node:internal/modules/cjs/loader:1005:19)
stderr:     at require (node:internal/modules/cjs/helpers:102:18)
stderr:     at requireLanguage (/website/node_modules/metalsmith-prism/lib/index.js:42:11)
stderr:     at Object.<anonymous> (/website/node_modules/metalsmith-prism/lib/index.js:86:11)
stderr:     at initialize.exports.each (/website/node_modules/cheerio/lib/api/traversing.js:300:24)
stderr:     at /website/node_modules/metalsmith-prism/lib/index.js:62:17
stderr:     at /website/node_modules/lodash/lodash.js:4967:15
stderr:     at baseForOwn (/website/node_modules/lodash/lodash.js:3032:24) {
stderr:   code: 'MODULE_NOT_FOUND',
stderr:   requireStack: [
stderr:     '/website/node_modules/metalsmith-prism/lib/index.js',
stderr:     '/website/build.js'
stderr:   ]
stderr: }