feature #21114 [Yaml] parse multi-line strings (xabbuh)

This PR was merged into the 3.3-dev branch.

Discussion
----------

[Yaml] parse multi-line strings

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

When working on #21084, I discovered that the YAML parser does not support (unquoted) multi-line strings (neither as plain strings nor as values for mappings). This PR adds support for them.

Commits
-------

ec593b923b [Yaml] parse multi-line strings

Parser.php

@@ -295,6 +295,42 @@ public function parse($value, $flags = 0)
                     return $value;
                 }
 
+                // try to parse the value as a multi-line string as a last resort
+                if (0 === $this->currentLineNb) {
+                    $parseError = false;
+                    $previousLineWasNewline = false;
+                    $value = '';
+
+                    foreach ($this->lines as $line) {
+                        try {
+                            $parsedLine = Inline::parse($line, $flags, $this->refs);
+
+                            if (!is_string($value)) {
+                                $parseError = true;
+                                break;
+                            }
+
+                            if ('' === trim($parsedLine)) {
+                                $value .= "\n";
+                                $previousLineWasNewline = true;
+                            } elseif ($previousLineWasNewline) {
+                                $value .= trim($parsedLine);
+                                $previousLineWasNewline = false;
+                            } else {
+                                $value .= ' '.trim($parsedLine);
+                                $previousLineWasNewline = false;
+                            }
+                        } catch (ParseException $e) {
+                            $parseError = true;
+                            break;
+                        }
+                    }
+
+                    if (!$parseError) {
+                        return trim($value);
+                    }
+                }
+
                 switch (preg_last_error()) {
                     case PREG_INTERNAL_ERROR:
                         $error = 'Internal PCRE error.';

Tests/ParserTest.php

@@ -1465,6 +1465,32 @@ public function testParseMultiLineUnquotedString()
 
         $this->assertSame(array('foo' => 'bar baz foobar foo', 'bar' => 'baz'), $this->parser->parse($yaml));
     }
+
+    public function testParseMultiLineString()
+    {
+        $this->assertEquals("foo bar\nbaz", $this->parser->parse("foo\nbar\n\nbaz"));
+    }
+
+    public function testParseMultiLineMappingValue()
+    {
+        $yaml = <<<'EOF'
+foo:
+- bar:
+    one
+
+    two
+    three
+EOF;
+        $expected = array(
+            'foo' => array(
+                array(
+                    'bar' => "one\ntwo three",
+                ),
+            ),
+        );
+
+        $this->assertEquals($expected, $this->parser->parse($yaml));
+    }
 }
 
 class B