fix: add several type checks to prevent crashes and seg faults - #676

Several native entry points assume their JavaScript arguments have the
expected type. When called with a wrong-typed value, they
abort the entire Node.js process through a failed C++ assertion or a segmentation fault.
A single bad argument from JavaScript takes down the whole process,
with no chance for a try/catch or a rejected promise to intervene.

This PR adds a type check to each affected entry point so the operation raises
a TypeError instead of crashing. Valid usage is unchanged.

As an indicative example, the following code results in a hard crash.

const libxml = require("libxmljs");

// A caller passes a number where a string or Buffer is expected.
libxml.parseXmlAsync(42).catch((err) => console.error(err));

# Assertion failed: val->IsArrayBufferView()
# ...
Aborted (core dumped)

After this change the same call rejects with a TypeError
(fromBufferAsync: buffer must be a string or a Buffer), which the .catch
handles normally.