add cmake build support

[ITotalJustice]

almost feature parity with the makefile builds, however still have to use makepkg manually to build ext4 and ntfs-3g.
i can use cmake command to do this but havent tried yet.

an example for how to use it:

cmake -B build -DCMAKE_TOOLCHAIN_FILE=${DEVKITPRO}/cmake/Switch.cmake -DUSBHSFS_GPL=ON -DUSBHSFS_EXAMPLE=ON
cmake --build build -j16

this will build the lib with ext4+ntfs and the example.

[ITotalJustice]

things left to do before merge

• release / debug builds (usbhsfsd and usbhsfs)
• document cmake build version in readme

[ITotalJustice]

I've merged the dev branch, fixed examples building, fixed finding the dkp installed gpl builds, added instructions to the readme.

to build normally:

cmake -B build -DUSBHSFS_GPL=ON -DUSBHSFS_EXAMPLES=ON 
cmake --build build -j

for including this into a project using cmake, i've added those instructions to the readme.