trivial: add .gitignore, compact CMakeLists.txt, forward declare library-dependent type

[kwvg]

Additional Notes

• contrib: add .gitignore
  • Contents taken from C++.gitignore, CMake.gitignore, Linux.gitignore, macOS.gitignore, Windows.gitignore courtesy of https://github.com/github/gitignore
  • Done to reduce annoyance
• build: resolve cmake warning by specifying minimum version on first line
  • Done to resolve warning

    CMake Warning (dev) at CMakeLists.txt:1 (project):
        cmake_minimum_required() should be called prior to this top-level project()
        call.  Please see the cmake-commands(7) manual for usage documentation of
        both commands.
    This warning is for project developers.  Use -Wno-dev to suppress it.
    

• build: remove duplicated sources list from CMakeLists
  • CMake was bumped due to the dependency on list transformers, which were employed to always ensure that CMake is supplied the absolute path to source files.
  • That feature was introduced in CMake 3.12
• refactor: don't expose libxml2 headers, forward declare types instead
  • In preparation for potential refactoring of the codebase into smaller, reusable libraries, headers should abstain from containing dependency-specific definitions, if possible
  • A dependent project shouldn't be concerned with what was used to build the library, assuming ideal circumstances. Keeping in line with that, forward declarations were employed to remove the libxml2 includes from the header.