make getTokensForParser return non-const

since data is allocated with an appender that is not cleared, GC owns
the memory and may be modified by the user if they wish to.

Can still be used in all places where const was used before.

src/dparse/lexer.d

@@ -555,7 +555,7 @@ public bool isLiteral(IdType type) pure nothrow @safe @nogc
  * `leadingTrivia` until there is the EOF, where it will be attached as
  * `trailingTrivia` again.
  */
-const(Token)[] getTokensForParser(R)(R sourceCode, LexerConfig config, StringCache* cache)
+Token[] getTokensForParser(R)(R sourceCode, LexerConfig config, StringCache* cache)
 if (is(Unqual!(ElementEncodingType!R) : ubyte) && isDynamicArray!R)
 {
     config.whitespaceBehavior = WhitespaceBehavior.include;