Add check for broken links

[Jack Bates (jablko)]

The list of dead links was harvested with the following:

git grep -h '^// \(Project\|Definitions\): ' 'types/*/index.d.ts' |
  sed '
    s/\r$//
    s%^// \(Project\|Definitions\): %%
    s/, /\n/g
  ' |
  sort --unique |
  while read -r url; do
    curl --fail --insecure --silent --url "$url" > /dev/null || echo "$url"
  done > broken-links.txt

A similar check was previously removed from dts-critic because it wasn't that useful. This one differs in that it only applies to unresolvable links. It's caught some typos, as well as link rot.

[Nathan Shively-Sanders (sandersn)]

1. I'm not sure I understand the intent. Is the broken-link error limited to the known list in broken-links.txt? Why not just fix those links then?
2. The did-you-mean-homepage error seems to behave the same as the former dts-critic error. Is that true? Won't it still break continually as random packages update their homepages on npm?