Implement accessibility.pluralize for Finnish

[SeSodesa]

This is a continuation of #88. The first commit implements pluralize but is there something else that needs to get done?

[SeSodesa]

The second commit moves dictionary definitions out of the pluralize function, just in case Typst allocates them during every function call.

[SeSodesa]

@Mc-Zen This might be in a usable state now. Let me know if there is something missing, although I cannot promise I can work on this after the next 30--40 minutes is gone.

[hpcfzl]

These entries sure add to the overall length, just as a suggestion have you thought about combining str.slice and the ssa/ssä suffixes as needed?

[Mc-Zen]

Yes, deriving the denominator unit versions from the plural versions could be done with a simple rule. However, I have a different problem with the denominator versions: As I understand, they also replace the per phrase. Currently, the output for m/s would be metriä jaettuna sekunnissa which I gather is wrong. Either we go with the simpler way (although not quite idiomatic to the language) or I could add some logic to remove the per phrase.

I can deal with this implementation but I have one question for @SeSodesa : what happens if multiple units are in the denominator? Like V/(m s), would that be voltti metrissä sekunnissa or does it get more complicated? If it gets more complicated, I would vote for the more generic version withthe per phrase.