Add more arrows to the standards

[samimaattaCelia]

These arrow characters are not in the standards, but can be inferred by following rules laid out in the standards:

Double arrows

sign ⇑ 34-34-156 # upwards double arrow (not in the standards)
sign ⇓ 16-16-156 # downwards double arrow (not in the standards)

sign ⇖ 126-35-35 # North West Double Arrow (not in the standards)
sign ⇗ 35-35-156 # North East Double Arrow (not in the standards)
sign ⇘ 26-26-156 # South East Double Arrow (not in the standards)
sign ⇙ 126-26-26 # South West Double Arrow (not in the standards)

Arrows

sign ↖ 126-35 # north west arrow (not strictly in the standards)
sign ↙ 126-26 # south west arrow (not strictly in the standards)

[samimaattaCelia]

These can be extrapolated from the standards and I have done so in commit 7d5ce70.