Add RbSys::ExtensionTask

[ianks]

This PR is a first cut for a rake-compiler-dock compatible extension task. For now, it just wraps the existing RCD extension task with saner defaults. Here is are the changes I made to wasmtime-rb to make integrate it:

diff --git i/rakelib/compile.rake w/rakelib/compile.rake
index 6df4c80..7695062 100644
--- i/rakelib/compile.rake
+++ w/rakelib/compile.rake
@@ -1,21 +1,7 @@
-require "rake/extensiontask"
+require "rb_sys/extensiontask"
 
-CROSS_PLATFORMS = [ENV["RUBY_TARGET"]].compact
-SOURCE_PATTERN = "**/src/**/*.{rs,toml,lock}"
-
-Rake::ExtensionTask.new("wasmtime_rb", GEMSPEC) do |ext|
+RbSys::ExtensionTask.new("wasmtime-rb", GEMSPEC) do |ext|
   ext.lib_dir = "lib/wasmtime"
-  ext.ext_dir = "ext"
-  ext.cross_compile = !CROSS_PLATFORMS.empty?
-  ext.cross_platform = CROSS_PLATFORMS
-
-  ext.cross_compiling do |gem_spec|
-    # No need for rb_sys to compile
-    gem_spec.dependencies.reject! { |d| d.name == "rb_sys" }
-
-    # Remove unnecessary files
-    gem_spec.files -= Dir[SOURCE_PATTERN, "**/Cargo.*", "**/extconf.rb"]
-  end
 end
 
 namespace :compile do

Much simpler. Users should begin adopting this interface, and in the future I have some other planned improvements for making cross-compilation much easier.

The other big other nice change is the root ./target directory will be used, will reduce duplicative compilation of crates.

[ianks]

Tested against wasmtime-rb -> https://github.com/bytecodealliance/wasmtime-rb/actions/runs/4095319708/jobs/7062155388

[ianks]

Confirmed working for commonmarker as well