Add cmake presets for Linux and Windows

Allowing building stuff using `cmake --preset ...` and not having to
specify so many parameters. Use `cmake --list-presets` to see all of
them.

The presets assume that you're building for your own system, so they
enable -march=native in release mode, export compiel commands and turn
on SOURCE_ASSETS. They always use `build` as the build directory and on
Windows `install` as the installation directory, but that can be
overridden on the command line. Windows assumes you're using vcpkg and
installed it into `C:\vcpkg` like the build instructions tell you to do.

The presets are generated using a script because the JSON format they
came up with doesn't have particularly great deduplication facilities
and we want to generate quite a lot of permutations.

Also extends .gitignore appropriately so that it will ignore any
directory with `install` in the name and the file
`CMakeUserPresets.json`, which can be used by the user to extend and
customize presets for their own purposes.

.gitignore

@@ -2,10 +2,12 @@
 Makefile
 Makefile.*
 CMakeLists.txt.user*
+CMakeUserPresets.json
 
 # Build directories
 /*build*/
-**/target
+**/target/
+/*install*/
 
 *.swp