LibRegex: Allow unknown escapes in non-unicode mode (for ECMA262)

This makes regexps like `/\x/` to work as normal.
Partially deals with #4189.

Libraries/LibRegex/RegexLexer.h

@@ -92,6 +92,7 @@ public:
     void back(size_t offset);
     void set_source(const StringView source) { m_source = source; }
     bool try_skip(char);
+    char skip();
 
     StringView slice_back(size_t offset) const { return m_source.substring_view(m_position - offset - 1, offset); }