Mark some methods as as impure for PHPStan

By default, PHPSTan considers assumes that all methods that return a
value are pure, meaning they have no side effects.

While we could configure PHPStan so it is consistent with Psalm, we do
not know how downstream projects are configured and should assume they
rely on the default behavior.

See https://phpstan.org/blog/remembering-and-forgetting-returned-values

Both moveNext() and glimpse() have side effects in that they modify
properties, and are therefore marked as impure.

src/AbstractLexer.php

@@ -164,6 +164,7 @@ public function isNextTokenAny(array $types)
      *
      * @return bool
      *
+     * @phpstan-impure
      * @psalm-assert-if-true !null $this->lookahead
      */
     public function moveNext()
@@ -220,6 +221,8 @@ public function peek()
      *
      * @return mixed[]|null The next token or NULL if there are no more tokens ahead.
      * @psalm-return Token<T, V>|null
+     *
+     * @phpstan-impure
      */
     public function glimpse()
     {