Implement the File API's open method, base File abstraction and File.stat operation.

[oleiade]

As part of #3141, implementing the agreed-upon design for k6's File API, we intend to:

1. declare a dedicated experimental k6 module
2. add an open function to it, as specified
3. add an almost empty vessel File abstraction so that the open method does return an object, albeit not functional as part of this iteration.
4. implement the File.stat method to retrieve file information about the file itself
5. implement the FileInfo abstraction to fulfill 👆

Details

In this implementation step, we should declare and implement the open function, as showcased in the dedicated design document. This function inputs a path string and returns a Promises resolving to a File instance. To give some minimal usefulness to the abstraction, we expect to have the File.stat method and its FileInfo construct counterparts exposed alongside it.

We expect the open function to perform all the underlying operations it should, regardless of whether we expose helpful methods on the resulting File object yet:

• resolve, clean, and secure the path to the resource
• ensure it exists and is accessible
• open it
• map the file descriptor and track it internally in the module, as described in the design doc, and in a fashion that allows us to fulfill the scope of the stat method.

The result would look something like this

open(path: string): Promise<File>

interface File {
	stat(): Promise<FileInfo>
}

interface FileInfo {
	name: string
	size: number
}