Expose more dictionary methods

[isabelgk]

It doesn't seem possible right now to get a count of keys in a dictionary (e.g. with max::dictionary_getentrycount()). It's possible from a subdict because you can get an atom_reference and then get the size of that, but it's not straightforward for a plain dict.

I'm sure there are more methods that would be useful to wrap: https://cycling74.com/sdk/max-sdk-8.2.0/group__dictionary.html

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From forums: https://cycling74.com/forums/accessing-nested-dictionaries-in-min/replies/1#reply-61afc274f0337245190dfa51

[testcase]

Here are some other items that should be exposed.
max::dictionary_getkeys_ordered maybe a method which returns std::vec with keys in it and calls max::dictionary_freekeys to free underlying handle.

max::dictionary_appendatoms by having a [](atoms a) operator ?

read and writing from json file.

would love a way to call a lambda on all entries in a dict.