Modernise Existing CMakeLists.txt

- Added to .gitignore CMakeUserPresets.json

### Configuration:
- Changed python command to use single quotes to make build output log more legible.
- Added GODOT_DEV_BUILD to allow differentiation of debug or Release builds.
- Added find logic for macos Cocoa library

### godot-cpp Changes
- godot-cpp-test is changed to be incorporated into the cmake build as a target.
- Duplicated godot-cpp target into [template_release, template_debug, editor]
- Created {platform}.cmake files mirroring the style of the SCons build.

CMake has a feature called generator expressions for its configuration variables that are evaluated at build time. This allows multi-configuration build systems to properly evaulate options. for msvc, xcode and nijna multi-config.

- Moved configuration options to generator expressions with the notable exclusion of OSX_ARCHITECTURES.
- Remove CMAKE_BUILD_TYPE from msvc CI target as Multi-Config generators ignore it

### godot-cpp-test Changes
- Removed majority of the cmake code, now that the godot-cpp project is setup, the majority of the flags will be propagated as transient dependencies
- Marked with EXCLUDE_FROM_ALL so that it isn't built as part of the 'all' target
- Updated ci to build the godot-cpp-test target from the root directory using cmake
- Tests passing for Windows, Linux, and Macos builds.

### Documentation
Updated with new information
Added Emscripten example
Added Android example

cmake/linux.cmake

@@ -0,0 +1,22 @@
+#[=======================================================================[.rst:
+Linux
+-----
+
+This file contains functions for options and configuration for targeting the
+Linux platform
+
+]=======================================================================]
+function( linux_options )
+    # Linux Options
+endfunction()
+
+function( linux_generate TARGET_NAME )
+
+    target_compile_definitions( ${TARGET_NAME}
+            PUBLIC
+            LINUX_ENABLED
+            UNIX_ENABLED
+    )
+
+    common_compiler_flags( ${TARGET_NAME} )
+endfunction()