AK: Exclude GenericLexer String APIs from the Kernel

These APIs are only used by userland, and String is OOM-infallible,
so let's just ifdef it out of the Kernel.

AK/GenericLexer.h

@@ -83,10 +83,12 @@ public:
         return true;
     }
 
+#ifndef KERNEL
     bool consume_specific(const String& next)
     {
         return consume_specific(StringView { next });
     }
+#endif
 
     constexpr bool consume_specific(const char* next)
     {
@@ -115,14 +117,18 @@ public:
     StringView consume_until(const char*);
     StringView consume_until(StringView);
     StringView consume_quoted_string(char escape_char = 0);
+#ifndef KERNEL
     String consume_and_unescape_string(char escape_char = '\\');
+#endif
 
     enum class UnicodeEscapeError {
         MalformedUnicodeEscape,
         UnicodeEscapeOverflow,
     };
 
+#ifndef KERNEL
     Result<u32, UnicodeEscapeError> consume_escaped_code_point(bool combine_surrogate_pairs = true);
+#endif
 
     constexpr void ignore(size_t count = 1)
     {
@@ -212,8 +218,10 @@ protected:
     size_t m_index { 0 };
 
 private:
+#ifndef KERNEL
     Result<u32, UnicodeEscapeError> decode_code_point();
     Result<u32, UnicodeEscapeError> decode_single_or_paired_surrogate(bool combine_surrogate_pairs);
+#endif
 };
 
 constexpr auto is_any_of(StringView values)