Add propane and initial skeleton grammar

Rsconscript

@@ -3,10 +3,25 @@ configure do
   check_cfg package: "libpng"
 end
 
-env do |env|
-  env["CCFLAGS"] += %w[-Wall -O2 -std=c++23]
-  env["CPPPATH"] += %w[src]
+class Propane < Builder
+  def run(options)
+    @sources += ["propane"] unless @sources.include?("propane")
+    if @command
+      finalize_command
+    else
+      command = %W[./propane -w --log #{@target}.log #{@sources.first} #{@target}]
+      standard_command("Propane <target>#{@target}<reset>", command, options)
+    end
+  end
+end
 
-  sources = glob("src/**/*.cpp")
+env do |env|
+  env.add_builder(Propane)
+  env["CCFLAGS"] += %w[-Wall -O2 -std=c++23]
+  env["CPPPATH"] += %W[src #{env.build_root}/src]
+
+  env.Propane("^/src/propane.cpp", "src/fart.propane")
+  env.barrier
+  sources = glob("src/**/*.cpp") + ["#{env.build_root}/src/propane.cpp"]
   env.Program("fart", sources)
 end