Add a way to retrieve version of C core being used

[szhorvat]

What is the feature or improvement you would like to see?

The igraph_version() R function should be able to return the C core version this R/igraph release is based on. This could be enabled by passing a parameter. This version can be obtained with the igraph_version() C function.

Use cases for the feature

Ease debugging for us. When people report an issue, I'd like to immediately see what version of the C core was used, without having to go digging in the git commit history.

References
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[maelle]

This looks like a good way for me to understand Stimulus better 😸

[szhorvat]

This is likely affected by #1259

[krlmlr]

#1229 is a draft (now closed), want to pick up from there.

[maelle]

@Antonov548 could you please help with this?

[maelle]

@krlmlr @schochastics maybe a good way to check we understand Stimulus

[szhorvat]

Just go ahead and give it a try. @krlmlr did the bulk of the work when creating the new vendoring setup. Now you can simply expose igraph_version, which does have an interface entry.