language: introduce the requirement to use return to return a value from a function

parser/src/main/kotlin/gay/pizza/pork/parser/Parser.kt

@@ -38,6 +38,7 @@ class Parser(source: TokenSource, attribution: NodeAttribution) :
       TokenType.Break -> parseBreak()
       TokenType.Continue -> parseContinue()
       TokenType.None -> parseNoneLiteral()
+      TokenType.Return -> parseReturn()
 
       else -> {
         throw ParseError(
@@ -332,6 +333,10 @@ class Parser(source: TokenSource, attribution: NodeAttribution) :
     PrefixOperation(ParserHelpers.convertPrefixOperator(it), parseExpression())
   }
 
+  override fun parseReturn(): Return = expect(NodeType.Return, TokenType.Return) {
+    Return(parseExpression())
+  }
+
   override fun parseSetAssignment(): SetAssignment = produce(NodeType.SetAssignment) {
     val symbol = parseSymbol()
     expect(TokenType.Equals)

parser/src/main/kotlin/gay/pizza/pork/parser/Printer.kt

@@ -154,6 +154,11 @@ class Printer(buffer: StringBuilder) : NodeVisitor<Unit> {
     visit(node.expression)
   }
 
+  override fun visitReturn(node: Return) {
+    append("return ")
+    visit(node.value)
+  }
+
   override fun visitSuffixOperation(node: SuffixOperation) {
     visit(node.reference)
     append(node.op.token)