Use rehype-highlight instead of remark-highlight.js & Update highlight theme to GitHub

[josemarluedke]

This will allow us to get the glimmer grammar for better highlight.

cc/ @NullVoxPopuli

Currently including highlightjs-glimmer breaks with the following error:

  - broccoliBuilderErrorStack: TypeError: Cannot read property 'length' of undefined
    at /Users/josemarluedke/code/oss/frontile/node_modules/highlight.js/lib/core.js:420:15
    at Array.map (<anonymous>)
    at join (/Users/josemarluedke/code/oss/frontile/node_modules/highlight.js/lib/core.js:414:18)
    at MultiRegex.compile (/Users/josemarluedke/code/oss/frontile/node_modules/highlight.js/lib/core.js:880:31)
    at ResumableMultiRegex.getMatcher (/Users/josemarluedke/code/oss/frontile/node_modules/highlight.js/lib/core.js:951:15)
    at ResumableMultiRegex.exec (/Users/josemarluedke/code/oss/frontile/node_modules/highlight.js/lib/core.js:972:22)
    at _highlight (/Users/josemarluedke/code/oss/frontile/node_modules/highlight.js/lib/core.js:2003:35)
    at Object.highlight (/Users/josemarluedke/code/oss/frontile/node_modules/highlight.js/lib/core.js:1626:9)
    at highlight (/Users/josemarluedke/code/oss/frontile/node_modules/lowlight/lib/core.js:48:17)
    at Object.highlightAuto (/Users/josemarluedke/code/oss/frontile/node_modules/lowlight/lib/core.js:94:15)
  - code: [undefined]
  - codeFrame: Cannot read property 'length' of undefined
  - errorMessage: Cannot read property 'length' of undefined
        at DocfyBroccoli

[NullVoxPopuli]

Is it just glimmer, or any language? My tests earlier broke on any language.

For glimmer, we may need to wait for highlightjs v11, and then lowlight to support it, and then rehype