Variation selectors

[T0mstone]

Some symbols have both a text style and an emoji style. These can be explicitly selected with Variation Selectors 15 and 16 (U+FE0E and U+FE0F), respectively.

Without these, there are currently (as of 35c18a1) the following duplicates between sym and emoji:

• sym.hash, emoji.hash
• sym.ast.basic, emoji.ast
• sym.copyright, emoji.copyright
• sym.trademark, emoji.tm
• sym.trademark.registered, emoji.reg
• sym.excl.{double, quest}, emoji.excl.{double, quest}
• sym.arrow.{l.r, t.b, tl, tr, bl, br, {l, r}.hook, {l, r, t, b}.filled, {t, b}.curve}, emoji.arrow.{l.r, t.b, tl, tr, bl, br, {l, r}.hook, {l, r, t, b}.filled, {t, b}.curve}
• sym.square.filled.big, emoji.square.black
• sym.square.filled.{tiny, small, medium}, emoji.square.black.{tiny, small, medium}
• sym.square.stroked.big, emoji.square.white
• sym.square.stroked.{tiny, small, medium}, emoji.square.white.{tiny, small, medium}
• sym.triangle.filled.{l, r}, emoji.triangle.{l, r}
• sym.ballot.check, emoji.ballot.check
• sym.suit.{spade, club, heart, diamond}.filled, emoji.suit.{spade, club, heart, diamond}
• sym.checkmark.heavy, emoji.checkmark.heavy
• sym.dash.wave.double, emoji.dash.wave.double

I haven't looked into which of the styles is the default for each of them, but we should probably give variation selectors to both sides to avoid ambiguity.

Prior work: #21, #23.