support figure space (U+2007)

[Werner LEMBERG]

I suggest that texinfo adds

```
\DeclareUnicodeCharacter{2007}{\hphantom{0}}
```

to support the figure space.  This would allow the emulation of
right-aligned number columns in tables, for example.

     1  foo
   101  bar

The nice thing is that it would work out of the box with HTML
browsers, too.  On the other hand, maybe there could be some further
massaging to convert U+2007 to an ordinary space entity together with
some formatting CSS so that cut-and-paste don't contain U+2007.


    Werner