Create `XS` Standard Library

[Kfeavel]

Standard library source code (such as bitmaps, linked lists, etc.) should be moved to an XS directory in the root level of the project. It will be compiled as a static library that can be linked into the kernel or into the unit tests.

The overall design should be something similar to Serenity OS's AK standard library. A __kernel__ and __xyris__ define will need to be added to the build process to help differentiate between code usable within the kernel, userspace, and
on other systems.

Additional Classes / Features to be added:

• SourceLocation (std::source_location)
• Vector (std::vector) (#295)
• String (std::string)