Support underscores in number literals

```
{{ 1000 == 1_000 ? 'yes' : 'no' }}
# now: syntax error
# this PR: "yes"
```

> As of PHP 7.4.0, integer literals may contain underscores (_) between digits, for better readability of literals. These underscores are removed by PHP's scanner.
https://www.php.net/manual/en/language.types.integer.php

This PR replicates this behaviour, using the regexp to match the literals and then remove the "_".

I'm targeting Twig4 but maybe 3.x would be ok?
I cannot think of a real case that
- does not trigger an error currently
- would work differently after this PR

src/Lexer.php

@@ -59,7 +59,7 @@ class Lexer
     public const STATE_INTERPOLATION = 4;
 
     public const REGEX_NAME = '/[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*/A';
-    public const REGEX_NUMBER = '/[0-9]+(?:\.[0-9]+)?([Ee][\+\-][0-9]+)?/A';
+    public const REGEX_NUMBER = '/[0-9]+(?:_[0-9]+)*(?:\.[0-9]+(?:_[0-9]+)*)?([Ee][\+\-][0-9]+)?/A';
     public const REGEX_STRING = '/"([^#"\\\\]*(?:\\\\.[^#"\\\\]*)*)"|\'([^\'\\\\]*(?:\\\\.[^\'\\\\]*)*)\'/As';
     public const REGEX_DQ_STRING_DELIM = '/"/A';
     public const REGEX_DQ_STRING_PART = '/[^#"\\\\]*(?:(?:\\\\.|#(?!\{))[^#"\\\\]*)*/As';
@@ -360,9 +360,9 @@ private function lexExpression(): void
         }
         // numbers
         elseif (preg_match(self::REGEX_NUMBER, $this->code, $match, 0, $this->cursor)) {
-            $number = (float) $match[0];  // floats
-            if (ctype_digit($match[0]) && $number <= \PHP_INT_MAX) {
-                $number = (int) $match[0]; // integers lower than the maximum
+            $number = (float) str_replace('_', '', $match[0]); // floats
+            if (ctype_digit(str_replace('_', '', $match[0])) && $number <= \PHP_INT_MAX) {
+                $number = (int) $number; // integers lower than the maximum
             }
             $this->pushToken(Token::NUMBER_TYPE, $number);
             $this->moveCursor($match[0]);

tests/Fixtures/expressions/underscored_numbers.test

@@ -0,0 +1,20 @@
+--TEST--
+Twig compile numbers literals with underscores correctly
+--TEMPLATE--
+{{ 0_0 is same as 0 ? 'ok' : 'ko' }}
+{{ 1_23 is same as 123 ? 'ok' : 'ko' }}
+{{ 12_3 is same as 123 ? 'ok' : 'ko' }}
+{{ 1_2_3 is same as 123 ? 'ok' : 'ko' }}
+{{ -1_2 is same as -12 ? 'ok' : 'ko' }}
+{{ 1_2.3_4 is same as 12.34 ? 'ok' : 'ko' }}
+{{ -1_2.3_4 is same as -12.34 ? 'ok' : 'ko' }}
+--DATA--
+return []
+--EXPECT--
+ok
+ok
+ok
+ok
+ok
+ok
+ok

tests/Fixtures/expressions/underscored_numbers_error.test

@@ -0,0 +1,8 @@
+--TEST--
+Twig does not allow to use 2 underscored between digits in numbers
+--TEMPLATE--
+{{ 1__2 }}
+--DATA--
+return []
+--EXCEPTION--
+Twig\Error\SyntaxError: Unexpected token "name" of value "__2" ("end of print statement" expected) in "index.twig" at line 2.