Code clean up

[qgustavor]

For a minor release:

1. Do checks to avoid the library completely crashing if it can't decrypt something (related to issue This library fails to decrypt some files in older accounts #218) and document it
2. Move _ prefixed method implementations elsewhere (like functions in module scope instead of in the class scope), like this one
3. Simplify promise->callback->promise chains (like here)
4. Fully document the code using TypeScript compatible JSDoc comments (I don't want a TypeScript rewrite for the same reasons Svelte switched from TypeScript to JSDoc, mainly, there are too many incompatible TypeScript versions in the wild [like Deno's vs official tsc]).
5. Remove all direct usage of MEGA's API outside an API translation class, as MEGA's API is full of abbreviations for methods and parameters that hurt code maintainability (idea from this comment).

For the next major release:

1. Remove the CJS build
2. Remove callbacks, change code to always use promises and async functions
3. Remove the AbortController polyfill (it looks like it's well supported nowadays)
4. Replace rsa.mjs with something better (it's use of a global state is not desirable and I guess Node.js already supports RSA natively)
5. Rename Files and Folders into Nodes to avoid collision with globalThis.File.

For a future major release:

1. Change the API from being based on classes to be programmatic in order to improve tree-shaking and allow for more custom workflows (based on those ideas).
2. Include platform specific code for Node.js, Deno (maybe it is pointless to support it since it's almost 100% Node.js compatible) and browsers to handle common cases of the library like:
   • Reading files from a system path (or from a File or Blob object in browsers)
   • Writing files from a system path (or using a[download] in browsers)