Add RawString methods

[ikorin24]

Summary

Add methods to U8Xml.RawString.
(IndexOf, RangeOf, LastIndexOf, LastRangeOf, Contains)

Public API Diff

 namespace U8Xml
 {
     public readonly struct RawString : IEquatable<RawString>
     {
+        public int IndexOf(byte value);
+        public DataRange RangeOf(char value);
+        public DataRange RangeOf(RawString value);
+        public DataRange RangeOf(ReadOnlySpan<byte> value);
+        public DataRange RangeOf(string value);
+        public DataRange RangeOf(ReadOnlySpan<char> value);
+        public int LastIndexOf(byte value);
+        public DataRange LastRangeOf(char value);
+        public DataRange LastRangeOf(RawString value);
+        public DataRange LastRangeOf(ReadOnlySpan<byte> value);
+        public DataRange LastRangeOf(string value);
+        public DataRange LastRangeOf(ReadOnlySpan<char> value);
+        public bool Contains(byte value);
+        public bool Contains(char value);
+        public bool Contains(RawString value);
+        public bool Contains(ReadOnlySpan<byte> value);
+        public bool Contains(string value);
+        public bool Contains(ReadOnlySpan<char> value);
     }
 }