validate HTML entities

[michaelficarra]

When I accidentally write &lte; when I mean ≤, ecmarkup should complain.

[bakkot]

&lte; is a particularly tricky one because - fun fact - &lte; decodes to <e;. Certain entities do not require a trailing semicolon, though this is a "missing-semicolon-after-character-reference parse error".

Should be possible to differentiate, but the package all the parsers use doesn't, so it might require a bit of work.