Fix Lexer Peek Bounding

Fixed an issue with lexer.peek() method that caused an unexpected panic caused from not checking for the condition that lex cursor is at the penultimate symbol

lexer.go

@@ -24,7 +24,8 @@ func (lexer *lexer) char() rune {
 // This look ahead is performed without moving the Lexer's cursor.
 // If the Lexer tap is exhausted, an EoF rune is returned.
 func (lexer *lexer) peek() rune {
-	if lexer.done() {
+	// If lexer is done or cannot peek, return EoF
+	if lexer.done() || lexer.cursor+1 == len(lexer.symbols) {
 		return rune(TokenEoF)
 	}