Add syntax highlighting to code tags

[Plastikmensch]

Closes #2011
Supersedes #2015

The old PR was a mess, so I redid it.

Overview of changes:

• Adds highlight.js node dependency
• Adds code-languages.yml containing supported languages to /config
• Changes MASTODON_STRICT and advanced text formatter to allow data-codelang attribute
• Adds data-codelang and title attributes to code tags in web UI

Preview:

Code blocks in the reply indicator are not highlighted as it uses a light background.
Unfortunately this is also the case for the report modal, where code blocks are highlighted.
Adding a background to code blocks would mitigate this problem, but it has been suggested multiple times, but not implemented.

code-languages.yml has been created with the output of the following script:

const highlightjs = require('highlight.js');

let langList = highlightjs.listLanguages();

let allLangs = '';
let count = 0;

for (let lang of langList) {
  if (lang.match(/^[a-zA-Z0-9.#+-]+$/)) allLangs += `- ${lang}\n`; count++;
  let langObject = highlightjs.getLanguage(lang).aliases;

  if(!langObject) continue;
  for (let alias of langObject) {
    if(alias.match(/^[a-zA-Z0-9.#+-]+$/)) allLangs += `- ${alias}\n`; count++;
  }
}

console.log(`Found ${count} valid languages and aliases:`);
console.log(allLangs);

This script has to be run again when adding third-party language support packages.

[Plastikmensch]

Just noticed that highlight.js now adds a "data-highlighted" attribute to highlighted elements.
Not sure whether to keep or remove that.

[github-actions]

This pull request has merge conflicts that must be resolved before it can be merged.

[github-actions]

This pull request has resolved merge conflicts and is ready for review.

[github-actions]

This pull request has merge conflicts that must be resolved before it can be merged.