Re-template json_pointer on string type (#3415)

* Make exception context optional

Change exception context parameter to pointer and replace context with
nullptr where appropriate.

* Support escaping other string types

* Add string concatenation function

Add variadic concat() function for concatenating char *, char, and
string types.

* Replace string concatenations using + with concat()

* Template json_pointer on string type

Change json_pointer from being templated on basic_json to being
templated on string type.

* Add unit test for #3388

Closes #3388.

* Fix regression test for #2958

* Add backwards compatibility with json_pointer<basic_json>

* Update json_pointer docs

* Allow comparing different json_pointers

* Update version numbers

doc/mkdocs/docs/api/json_pointer/string_t.md

@@ -0,0 +1,12 @@
+# <small>nlohmann::json_pointer::</small>string_t
+```cpp
+using string_t = RefStringType;
+```
+
+The string type used for the reference tokens making up the JSON pointer.
+
+See [`basic_json::string_t`](../basic_json/string_t.md) for more information.
+
+## Version history
+
+- Added in version 3.11.0.