Stop relying on the filesystem in the core library

[Angelmmiguel]

Currently, the core library can load a SVG file directly from the filesystem. The main purpose of the core library is to read a SVG content and extract information from it. The load of the SVG content from the filesystem is not directly related to the core feature and it can be done from the CLI side.

Using fs in the core library is preventing other interesting use cases for SVGI as the core library can be used in web apps too. An example of this limitation is #19.

After removing this dependency, the core library will be valid for the browser environment.