tdf#120047 Support attribute harpoon/wideharpoon in Math

[Takeshi Abe]

Hi,

To solve tdf#120047 [1], I am going to add a couple of new attributes
harpoon/wideharpoon to the starmath command syntax [2].
At the same time the patch introduces a new glyph to OpenSymbol font
to make sure that the harpoon symbol (U+20D1) has the consistent width
with COMBINING RIGHT ARROW ABOVE (U+20D7), which is the one shown
as the arrow for attributes vec/widevec so far.
Review and suggestions are highly appreciated.

[1] https://bugs.documentfoundation.org/show_bug.cgi?id=120047
[2] https://gerrit.libreoffice.org/#/c/72987/

Cheers,
-- Takeshi Abe