Move method implementations of `HTMLString` into `StringProtocol`.

[es-kumagai]

When we get a substring value from a String value (using .prefix, .suffix, subscript and so on), we can get the value in Substring type. The Substring type provide a way to access the substring value without copying the part of string from original value. So, to use Substring value directly, we can use the substring more efficiently.

For that reason, I tried to move the method implementations of HTMLString from String type to StringProtocol protocol. The StringProtocol is conformed to both String type and Substring type. So, to implementing the methods by StringProtocol instead of String, the methods will belong both String and Substring. This allows us to use HTMLString methods by Substring directly without copying the part of content.