Added digit emojis 0 to 9

[Liggliluff]

Added emojis for digits 0 to 9, using the base digits followed by variation selector-16.

Digits 0 to 9 has emoji variants, available in some of the emoji fonts. Can be seen on Emojipedia:
https://emojipedia.org/digit-zero/ .. https://emojipedia.org/digit-nine/

These files has been created by taking the keycap emojis; removed the background and made the text grey, followed by enlarging it to the size of the White Exclamation Mark (U+2755).

[CLAassistant]

All committers have signed the CLA.

[Mennaruuk]

These numerical digits would be a wonderful addition to Twemoji!!! Will color stay gray? It won't be a problem with light mode, but with dark mode it could be difficult to see.

[Liggliluff]

@Mennaruuk These have the same colour as the other symbols, like the question mark and exclamation mark. I think there's a higher contrast in the dark mode than light mode, so it'll rather be harder to see in light mode.

To ensure that, I did a contrast check. Against a white background it's 1.48 (bad score), and against Twitter's very dark blue background in dark mode, it's 12.76 (extremely good score). So these grey symbols should rather be darker.

[Mennaruuk]

@Mennaruuk These have the same colour as the other symbols, like the question mark and exclamation mark. I think there's a higher contrast in the dark mode than light mode, so it'll rather be harder to see in light mode.

To ensure that, I did a contrast check. Against a white background it's 1.48, and against Twitter's very dark blue background in dark mode, it's 12.76. So these grey symbols should rather be darker.

Wow! The more you know. Can't wait for these to come to Twemoji! Thanks for your hard work :)