breaking: Rework CreateCommandOpts, CommandInfos and CommandComplete

[c-c-k]

PR change list

• Add custom serde deserializer for CommandComplete.
• Rename CommandComplete::CustomList(fun) to CommandComplete::Callback(fun).
• Add CommandComplete::Custom(String).
• Add CommandComplete::Customlist(String).
• Add other CommandComplete variants listed in the NVIM documentation.
• Add CommandComplete tests.
• Change CommandInfos::complete's type from String to CommandComplete.
• Mark nvim_oxi::api::get_commands as unsafe.
• Modify CreateCommandOpts::complete to accept a function directly.
• Modify existing create_user_command tests to get_commands being unsafe.

Rationale

This PR was originally necessary to fix the failing create_del_user_command and user_command_with_count tests because NVIM v0.12 modified a few of the builtin plugin commands to define the complete field directly as a Lua function instead of as a "customlist" with a complete_arg="v:lua..." field.

In the process of working on this PR however I realized that the C API nvim_get_commands function creates a Lua registry slot for each command and completion callback, which it expects the caller to clean up.
Currently nvim-oxi does not perform this cleanup.
Consequentially calling nvim_oxi::api::get_commands creates a serious leak of Lua registry slots which in turn also prevents the callbacks from being garbage collected if their commands are deleted.

Fixing this will probably require reworking nvim_oxi::Function into freeing the Lua registry slot it contains a reference to when dropped.
This in turn will require adjusting the behavior of all the nvim_oxi functions that use Function, which is beyond the scope of this PR.
However, since the above solution will require converting all the raw lua_refs returned by nvim_get_commands into Function's it would be necessary to process the entire Dictionary returned by the C API function before returning it to the user.
In which case I think it also makes sense to change the return type of nvim_get_commands from impl SuperIterator<CommandInfos> to HashMap<String, CommandInfos> which would also fall in line with the return type of the corresponding functions in the C and Lua API's.

edit: After some AI consultation I realize a better solution would probably be to create a custom iterator that would do minimal cleanup processing for any remaining items in the iterator when it's dropped.

Regardless of the above, I think it would be appropriate to rename CommandComplete::CustomList to CommandComplete::Callback so as to use CommandComplete::Customlist for it's official purpose of holding the name of a Vim Script function.

Also I think it would be convenient to allow CreateCommandOpts::complete to accept a function directly (but still to also accept CommandComplete).

[c-c-k]

Will drop the v0.10 flags, sec