Allow using $$ instead of # in preprocessor directives

docs/lang/preprocessor.md

@@ -16,6 +16,9 @@ Despite its similarity to the C preprocessor, it's much more restricted in its p
 
 * separate namespaces for the preprocessor and the language (you need to use `#use` to use a preprocessor constant in the code)
 
+Preprocessor directives by default start with `#`.
+To avoid conflicts with C preprocessor (for users wishing to use it), it is also possible to replace `#` with `$$`.
+
 ### Defining feature values
 
 Feature values are defined in the `[define]` section of the platform definition file.  

src/main/scala/millfork/parser/Preprocessor.scala

@@ -15,7 +15,7 @@ case class PreprocessingResult(source: String, featureConstants: Map[String, Lon
 
 object Preprocessor {
 
-  private val Regex = raw"\A\s*#\s*([a-z]+)\s*(.*?)\s*\z".r
+  private val Regex = """\A\s*(?:#|\$\$)\s*([a-z]+)\s*(.*?)\s*\z""".r
 
   def preprocessForTest(options: CompilationOptions, code: String): PreprocessingResult = {
     apply(options, "", code.lines.toSeq)

src/test/scala/millfork/test/BasicSymonTest.scala

@@ -196,7 +196,7 @@ class BasicSymonTest extends FunSuite with Matchers {
         | #use ARCH_I80
         |
         | #use HORSE = ARCH_6502
-        | #use COW = ARCH_I80
+        | $$use COW = ARCH_I80
         |
         | #if 1
         | asm void main () {