Json update + static analysis

[McCallisterRomer]

• Including json headers directly instead of through fetch_content. With fetch_content, we must do a cmake install to make the headers available to NcEngine, which clutters up the install directory over there with files we don't care about. There's a recommended way to fetch just the header-only stuff, but it seems to be broken.
• Adding msvc static analyzer and bumping warnings to /W4
• Added a few warnings for mingw.
• Fixing stuff found with higher warnings.