generateMnemonic method can take conflicting arguments

[thunderbiscuit]

It is currently possible to create a mnemonic using the following code:

var res = await generateMnemonic(
    wordCount: WordCount.WORDS12,
    entropy: Entropy.ENTROPY256
);

Which are incompatible inputs.

[thunderbiscuit]

I think in this case you don't need the entropy argument as provided (it's a 1-to-1 match to the number of words, so they express the same concept).

One thing I could see eventually being added is an entropy argument where you actually provide the starting entropy (the type would be ByteArray or something like that on the Dart side if the user has that entropy saved in a file or something), but that's a very advanced user workflow.