feat: add line comment support to lexer with skipLineComment helper

Add a new `skipLineComment` function to the compiler's lexer that consumes
characters from the current position until a newline or end-of-file is
encountered. Update the `readRawToken` switch case for '/' to detect the
"//" sequence and delegate to the new function instead of emitting a
TOKEN_SLASH. Also add a line comment to the example hello.wren file to
demonstrate the feature.

example/hello.wren

@@ -1,4 +1,4 @@
-class Foo {
+class Foo { // line comment
   bar {
     123
   }