Add vnu.jar support

[XhmikosR]

This is WIP because there are some things that need to be addressed:

1. anchor-markdown-headings.js: fix IDs to be unique. #2441 needs to be merged
2. Convert a few posts to Markdown #2437 probably needs to be merged too
3. There's a bug in vnu.jar, I have already discussed with sideshowbarker (not CC'ing right now), that causes the build to exit with non-zero.
4. I'd prefer it if we could actually fix the table errors, because these are real issues:

   "file:/home/travis/build/XhmikosR/nodejs.org/build/en/download/index.html":276.50-277.32: error: Table column 3 established by element "td" has no cells beginning in it.
   "file:/home/travis/build/XhmikosR/nodejs.org/build/en/download/index.html":277.113-278.32: error: Table column 7 established by element "td" has no cells beginning in it.
   

5. The license of the new file might need to be adapted for this repo. It's just a simple script so it's OK to change it, especially since I also adapted it to fit our needs

Note that this is how I noticed and fixed many of the issues so far, like bae664a, b533fcd, a714afc, ceee600 etc.

IMO this should definitely land as soon as we sort out the above points. Personally I'd remove htmllint too; I'm having issues with it on Windows and it just doesn't seem to catch any real issues.

Marking it as draft for further discussion and tweaks.

vnu.jar: https://github.com/validator/validator

Closes #2398

[XhmikosR]

I think we should try fixing the table errors. I'm gonna revert the ignore so that we see the errors, and someone might be able to help fix those. I believe the wrong exit code comes from this for some reason.

[XhmikosR]

Alright, I don't know how to fix the table errors, so I have them ignored for now.

This is ready for review. Note that on Windows there will be a failure due to #2422. This isn't triggered on nix, but I've yet to figure out the root cause (like I never managed to figure it out for the slashes, hence the workaround we have).

[XhmikosR]

After #2511 CI will be green. Generally that is why I insist that branches need to be up to date and we have the button.

[XhmikosR]

Can we move with this?

[XhmikosR]

Can I get some reviews please?

This is the only proper way to make sure we are valid and we don't have issues like duplicate IDs and so on. Note that I intentionally skip the test if Java isn't available, and that I haven't added the script in npm t like I haven't done it with linkinator, because these tests are a little slow.

On CI Java is present so this will run there.

[Trott]

@nodejs/website

[phillipj]

LGTM