detect broken links

[Fil]

Currently only warning, but we could make it break the build like we do in Plot's documentation. Should this be a build option?

closes #363
closes #1683

(Test by running yarn docs:build. Merge #1699 to see the happy state.)

happy

unhappy

[Fil]

The page sizes table we show at the end and the link checker both look similar in that they are "reporters" rather than strictly necessary for the build.

I put the link checker at this location because it's fast and if it must break the build it's better to do it sooner. But I'd prefer the happy report to be shown at the end.

[mbostock]

Hooray! Great to have this.

I made some changes. To summarize:

• I encapsulated the code a bit more, reducing the number of local variables in the already quite large and stateful build function. It would be nice to have separate unit tests for validateLinks, perhaps, but it’s pretty simple so not urgent.
• I moved the broken link warning to the end where it’s more likely to be seen. If we want to make this a hard error, then we can move it earlier.
• I renamed links to localLinks to indicate that we’re only extracting (and validating) local links, not all links.
• I added encoding as appropriate (${path}#${encodeURIComponent(id)}).
• I skipped validation of <a href download> since these links are rewritten to point to files.
• I disallowed (non-download) links to files, since these links are not rewritten and would be broken.
• I would like to allow links to exported files, but I was lazy and didn’t implement this yet.
• I allowed links to exported modules (which is unlikely but should work).
• I added normalization for trailing .html (in addition to the existing /index).
• I used parseRelativeUrl instead of hacking URL with proto:….
• I fixed a longstanding quirk in resolvePath (and added tests).
• I added unit tests for the new findAssets logic.