Add jssyntax as Prism alias for syntax boxes

[teoli2003]

See mdn/mdn-community#149.

This adds an alias in Prism for js: jssyntax.

The plan is to use it with syntax boxes (Web APIs and JS) that have pseudo-JS. We want them to be highlighted as JavaScript by Prism, but distinct so that tools like Prettier and ESLint can skip them.

[OnkarRuikar]

We may also need alias jsonc for json. [ref]

[teoli2003]

I'll prefer to wait a bit for jsonc and see how it turns out with this one first, even if it is easy to reverse.

We can use Prettier/ESLint on js blocks first, then extend to json.

[lionralfs]

Prism already supports comments in JSON: PrismJS/prism#1595
For a live example see https://developer.mozilla.org/en-US/docs/MDN/Structures/Compatibility_tables#basic_compat_data_structure

It seems that the only reason this PR is not merged yet is to make sure Prettier/ESLint can handle jsonc. With JSON not currently being linted anyways, wouldn't it make sense to merge this to address issues in https://github.com/orgs/mdn/discussions/158 first

[teoli2003]

I would prefer for this to land, and to keep the json/jsonc discussion separated from this PR.

[teoli2003]

To @OnkarRuikar and @lionsralf: once this lands, let's test it on 20-30 pages and only when we are sure that this is working as expected, without any surprise or side-effect, we will be updating the many pages with a syntax.

Let's be cautious. I'm very confident, but let's be cautious.

[OnkarRuikar]

I've tried it in my local and it works as expected:

before

after