AK: Allow escaping of keys in SourceGenerator

This allows the opening and closing characters of the SourceGenerator
to be used in the source text to be used for purposes other than keys.

(cherry picked from commit 634f2f655b7038d8f487ed94cd4f4e087a5a842c)

AK/SourceGenerator.h

@@ -1,6 +1,7 @@
 /*
  * Copyright (c) 2020, the SerenityOS developers.
  * Copyright (c) 2023, Sam Atkins <[email protected]>
+ * Copyright (c) 2024, Tim Ledbetter <[email protected]>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -21,17 +22,19 @@ class SourceGenerator {
 public:
     using MappingType = HashMap<StringView, String>;
 
-    explicit SourceGenerator(StringBuilder& builder, char opening = '@', char closing = '@')
+    explicit SourceGenerator(StringBuilder& builder, char opening = '@', char closing = '@', char escape = '\\')
         : m_builder(builder)
         , m_opening(opening)
         , m_closing(closing)
+        , m_escape(escape)
     {
     }
-    explicit SourceGenerator(StringBuilder& builder, MappingType&& mapping, char opening = '@', char closing = '@')
+    explicit SourceGenerator(StringBuilder& builder, MappingType&& mapping, char opening = '@', char closing = '@', char escape = '\\')
         : m_builder(builder)
         , m_mapping(move(mapping))
         , m_opening(opening)
         , m_closing(closing)
+        , m_escape(escape)
     {
     }
 
@@ -70,7 +73,20 @@ public:
         GenericLexer lexer { pattern };
 
         while (!lexer.is_eof()) {
-            m_builder.append(lexer.consume_until(m_opening));
+            m_builder.append(lexer.consume_until([&](char ch) { return ch == m_opening || ch == m_escape; }));
+            if (lexer.consume_specific(m_escape)) {
+                if (!(lexer.next_is(m_opening) || lexer.next_is(m_escape))) {
+                    if (lexer.is_eof())
+                        warnln("Unexpected EOF while parsing escape sequence on SourceGenerator");
+                    else
+                        warnln("Invalid escape sequence found `{}{}` on SourceGenerator", m_escape, lexer.peek());
+
+                    VERIFY_NOT_REACHED();
+                }
+
+                m_builder.append(lexer.consume());
+                continue;
+            }
 
             if (lexer.consume_specific(m_opening)) {
                 auto const placeholder = lexer.consume_until(m_closing);
@@ -129,7 +145,9 @@ public:
 private:
     StringBuilder& m_builder;
     MappingType m_mapping;
-    char m_opening, m_closing;
+    char m_opening { '@' };
+    char m_closing { '@' };
+    char m_escape { '\\' };
 };
 
 }