Use correct syntax for assertions

The previous syntax resulted in assertions having an impact on both
cases when true and false were returned.
Let us switch to equality assertions when we cannot deduce anything for
the other case, and to a single assertion when we can deduce something
in both cases.

See https://psalm.dev/docs/annotating_code/assertion_syntax/#equality-assertions
See vimeo/psalm#8896

src/AbstractLexer.php

@@ -148,7 +148,7 @@ public function getInputUntilPosition($position)
      *
      * @return bool
      *
-     * @psalm-assert-if-true !null $this->lookahead
+     * @psalm-assert-if-true !=null $this->lookahead
      */
     public function isNextToken($type)
     {
@@ -162,7 +162,7 @@ public function isNextToken($type)
      *
      * @return bool
      *
-     * @psalm-assert-if-true !null $this->lookahead
+     * @psalm-assert-if-true !=null $this->lookahead
      */
     public function isNextTokenAny(array $types)
     {
@@ -175,7 +175,6 @@ public function isNextTokenAny(array $types)
      * @return bool
      *
      * @psalm-assert-if-true !null $this->lookahead
-     * @psalm-assert-if-false null $this->lookahead
      */
     public function moveNext()
     {