Consider more semantic-neutral tag names

[matklad]

This is super-weak nice to have, but we have, eg code_block and emoji tag names, which are specific to one use case. Given the extensibility goal of djot, we might want to go for more general names, like verbatim_block and subst.

[jgm]

Agreed. I've been trying to think of a good name to use instead of emoji.
Not suer about subst.

[matklad]

symbol as another alternative. 🤔 actually, as :thinking: is an alternative way to write 🤔, alt might also work.

[jgm]

I like symbol.