LibWeb: Move Fetch::collect_an_http_quoted_string() into HTTP.{cpp,h}

The Fetch spec is too big to have a generic AbstractOperations.{cpp,h}
file, so let's keep AOs in their section-specific files.

Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP.cpp

@@ -0,0 +1,81 @@
+/*
+ * Copyright (c) 2022, Luke Wilde <lukew@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/GenericLexer.h>
+#include <AK/String.h>
+#include <AK/StringBuilder.h>
+#include <LibWeb/Fetch/Infrastructure/HTTP.h>
+
+namespace Web::Fetch {
+
+// https://fetch.spec.whatwg.org/#collect-an-http-quoted-string
+String collect_an_http_quoted_string(GenericLexer& lexer, HttpQuotedStringExtractValue extract_value)
+{
+    // To collect an HTTP quoted string from a string input, given a position variable position and optionally an extract-value flag, run these steps:
+    // 1. Let positionStart be position.
+    auto position_start = lexer.tell();
+
+    // 2. Let value be the empty string.
+    StringBuilder value;
+
+    // 3. Assert: the code point at position within input is U+0022 (").
+    VERIFY(lexer.peek() == '"');
+
+    // 4. Advance position by 1.
+    lexer.ignore(1);
+
+    // 5. While true:
+    while (true) {
+        // 1. Append the result of collecting a sequence of code points that are not U+0022 (") or U+005C (\) from input, given position, to value.
+        auto value_part = lexer.consume_until([](char ch) {
+            return ch == '"' || ch == '\\';
+        });
+
+        value.append(value_part);
+
+        // 2. If position is past the end of input, then break.
+        if (lexer.is_eof())
+            break;
+
+        // 3. Let quoteOrBackslash be the code point at position within input.
+        // 4. Advance position by 1.
+        char quote_or_backslash = lexer.consume();
+
+        // 5. If quoteOrBackslash is U+005C (\), then:
+        if (quote_or_backslash == '\\') {
+            // 1. If position is past the end of input, then append U+005C (\) to value and break.
+            if (lexer.is_eof()) {
+                value.append('\\');
+                break;
+            }
+
+            // 2. Append the code point at position within input to value.
+            // 3. Advance position by 1.
+            value.append(lexer.consume());
+        }
+
+        // 6. Otherwise:
+        else {
+            // 1. Assert: quoteOrBackslash is U+0022 (").
+            VERIFY(quote_or_backslash == '"');
+
+            // 2. Break.
+            break;
+        }
+    }
+
+    // 6. If the extract-value flag is set, then return value.
+    if (extract_value == HttpQuotedStringExtractValue::Yes)
+        return value.to_string();
+
+    // 7. Return the code points from positionStart to position, inclusive, within input.
+    auto position = lexer.tell();
+    auto number_of_characters_to_consume = position - position_start + 1;
+    lexer.retreat(number_of_characters_to_consume);
+    return lexer.consume(number_of_characters_to_consume);
+}
+
+}