build: add cmake support

[ksqsf]

Add support for building with CMake. See INSTALL.md for usage.

Demo build:

make -Clibrime deps
bash librime/install-plugins.sh hchunhui/librime-lua lotem/librime-octagram rime/librime-predict
cmake -S. -GNinja -Bbuild -DSIGN_KEY='localrun'
ninja -Cbuild install

The xcode build system is preserved, and I do not expect cmake to be the official build system due to the lack of universal binaries.

Some advantages:

1. Simple: everything (except librime deps and plugins) is now packed into the cmake scripts and there's no need to call different make targets and scripts. Even a novice can build squirrel in minutes.
2. Performance: CMake+Ninja is tremendously faster than xcodebuild.
3. Don't need much maintenance: the xcode build system requires hard-coding file list, but cmake can automatically find them. We don't need to pay much attention to cmake once it's there.
4. ninja install Just Works™: squirrel won't disappear from the IME list.

[ksqsf]

Building swift code with cmake still leaves much to be desired and I don't have the motivation to improve it.

Closing this in favor of a potentially better tool: meson or xmake.