refactor: use assert_character() instead of as.character()

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[szhorvat]

I'm not experienced enough with R to be able to follow the code without considerable time investment, but I wanted to point out that in many cases, numerical vertex IDs or vertex names are auto-converted to characters. This is convenient and probably desirable. I do not know if this change breaks any of the relevant use cases, but it's good to pay attention here.

[maelle]

@szhorvat thanks! Yes, I'm hoping the revdeps checks would help with that. If this breaks too many things, I won't merge it.

[szhorvat]

And just another reminder that our users are not just dependent packages, but also individuals who use igraph for research, often interactively. Unfortunately it is difficult to track this second kind of user, even though IMO they are the majority.