add support for dnd5e v3 and foundry v12#236

This PR along with [this one](https://github.com/zeel01/input-expressions/pull/11) adds support for dnd5e system v3 and foundry v12. - `_onItemCreate` has been removed, so I had to add a custom handler. - All other handlers for skills, saves, etc weren't working either, so I added custom handlers as well. Not sure if it's the best approach, but couldn't get it to work otherwise and didn't see any methods from the base 5e sheet that could be used for this. - I had to add a `getLanguageChecklist` method, because the shape of `CONFIG.DND5E.languages` has changed. It does not have a label (but the others such as damage, conditions, etc, do) and it can contain `children` for subgroups. - dnd5e v3 enrichers seem to be working fine just by calling `TextEditor.enrichHTML` asynchronously in `getData`. - I updated some calls to `mergeObject` and `flattenObjects` to avoid deprecation warnings. As far as I can tell everything I tested seems to be working now correctly in foundry v12.328 and dnd5e v3.2.1.