Support Djot-like symbols?

[Omikhleia]

Djot has this concept of a symbol, as a word surrounded by colons (:symb:)... which could be an emoji or something else, it's currently left to the decision of the renderer...

Heh, perhaps we could have this in Lunamark too, behind a djot_symbols extension?

(Full disclosure: I am seriously considering toying with it in rather arcane and likely unintended ways in my Djot support for SILE, just 'cause I kind of like the idea of variable substitution without external preprocessing)