Add support for file versions

[qgustavor]

Calling .upload() on a file overrides it adding a new version.

Missing:

• Handle naming: it should use the original file name I guess.
• Handle how revisions work: at the moment the library is unaware of revisions and seems to always download the newest one.
• Testing: testing on the real server, then implementing the server logic in mega-mock, then writing tests, then testing on real server again

[qgustavor]

I don't like this API: if we change .upload() to handle file revisions, it makes it messier. Some options I considered/consider:

• Call .upload() on the parent folder, provide the overridden file as a parameter. It was the initial implementation, but seemed too not intuitive.
• Call .upload() on the file, which is the current implementation, but you need to provide a file name on the first parameter anyway, which is not intuitive again.
• Changing the current implementation to not require the first parameter in case of a file is being overridden can cause some TypeScript typing shenanigans I prefer to avoid.
• Create an .uploadRevision() method instead? Like with .link() and .shareFolder(), one can just call the other method internally, but avoiding making the API non-intuitive.

[qgustavor]

Implemented uploadVersion and changed the wording to match the original MEGA clients. The current implementation avoids the naming problem while being more intuitive. The current code is still unaware of file versions.