Fix unknown escape sequences in string literals to preserve backslash

Previously, when the lexer encountered an unknown escape sequence like
\S in a string, it would only write the character after the backslash,
losing the backslash itself. This caused strings like 'Win\Sys' to be
stored as 'WinSys' instead of 'Win\Sys'.

This fix preserves both the backslash and the following character for
unknown escape sequences, matching ClickHouse's behavior.

Enables 5 passing tests.

Author: claude

lexer/lexer.go

@@ -397,7 +397,8 @@ func (l *Lexer) readString(quote rune) Item {
 				val := hexValue(hex1)*16 + hexValue(hex2)
 				sb.WriteByte(byte(val))
 			default:
-				// Unknown escape, just write the character after backslash
+				// Unknown escape, preserve both the backslash and the character
+				sb.WriteRune('\\')
 				sb.WriteRune(l.ch)
 			}
 			l.readChar()

parser/testdata/01497_extract_all_groups_empty_match/metadata.json

@@ -1 +1 @@
-{"todo": true}
+{}

parser/testdata/01847_bad_like/metadata.json

@@ -1 +1 @@
-{"todo": true}
+{}

parser/testdata/02374_regexp_replace/metadata.json

@@ -1 +1 @@
-{"todo": true}
+{}

parser/testdata/02580_like_substring_search_bug/metadata.json

@@ -1 +1 @@
-{"todo": true}
+{}

parser/testdata/03031_input_format_allow_errors_num_bad_escape_sequence/metadata.json

@@ -1 +1 @@
-{"todo": true}
+{}