Support to compile *.metal files to generate *.metalib and improve xcode.application rule

[waruqi]

Add xcode.metal rule

We can use add_rules("code.metal") rule to compile *.metal files and generate xxx.metallib in target directory

target("test")
    add_rules("xcode.metal")
    add_files("src/*.metal")
    add_files("src/*.m")
    add_frameworks("MetalKit")

Improve xcode.application rule

this rule also contain xcode.metal rule automatically.

We can also build *.metal files to add it as default.metallib when we build macOS/iOS application.

so we can use default metal library in application.

example project: https://github.com/xmake-io/xmake/blob/dev/tests/projects/objc/metal_app/xmake.lua

add_rules("mode.debug", "mode.release")

target("HelloTriangle")
    add_rules("xcode.application")
    add_includedirs("Renderer")
    add_frameworks("MetalKit")
    add_mflags("-fmodules")
    add_files("Renderer/*.m", "Renderer/*.metal")
    if is_plat("macosx") then
        add_files("Application/main.m")
        add_files("Application/AAPLViewController.m")
        add_files("Application/macOS/Info.plist")
        add_files("Application/macOS/Base.lproj/*.storyboard")
        add_defines("TARGET_MACOS")
        add_frameworks("AppKit")
    elseif is_plat("iphoneos") then
        add_files("Application/*.m")
        add_files("Application/iOS/Info.plist")
        add_files("Application/iOS/Base.lproj/*.storyboard")
        add_frameworks("UIKit")
        add_defines("TARGET_IOS")
    end

$ xmake
[  9%]: compiling.xcode.release Application/iOS/Base.lproj/LaunchScreen.storyboard
[  9%]: compiling.xcode.release Application/iOS/Base.lproj/Main.storyboard
[  9%]: processing.xcode.release Application/iOS/Info.plist
[ 18%]: ccache compiling.release Renderer/AAPLRenderer.m
[ 18%]: ccache compiling.release Application/AAPLViewController.m
[ 18%]: ccache compiling.release Application/AAPLAppDelegate.m
[ 18%]: ccache compiling.release Application/main.m
[ 18%]: compiling.metal Renderer/AAPLShaders.metal
[ 81%]: linking.metal default.metallib
[ 81%]: linking.release HelloTriangle
[ 90%]: generating.xcode.release HelloTriangle.app
[100%]: build ok!

$ xmake run