json start/end position implementation (#4517)

* Add implementation to retrieve start and end positions of json during parse

* Add more unit tests and add start/stop parsing for arrays

* Add raw value for all types

* Add more tests and fix compiler warning

* Amalgamate

* Fix CLang GCC warnings

* Fix error in build

* Style using astyle 3.1

* Fix whitespace changes

* revert

* more whitespace reverts

* Address PR comments

* Fix failing issues

* More whitespace reverts

* Address remaining PR comments

* Address comments

* Switch to using custom base class instead of default basic_json

* Adding a basic using for a json using the new base class. Also address PR comments and fix CI failures

* Address decltype comments

* Diagnostic positions macro (#4)

Co-authored-by: Sush Shringarputale <sushring@linux.microsoft.com>

* Fix missed include deletion

* Add docs and address other PR comments (#5)

* Add docs and address other PR comments

---------

Co-authored-by: Sush Shringarputale <sushring@linux.microsoft.com>

* Address new PR comments and fix CI tests for documentation

* Update documentation based on feedback (#6)

---------

Co-authored-by: Sush Shringarputale <sushring@linux.microsoft.com>

* Address std::size_t and other comments

* Fix new CI issues

* Fix lcov

* Improve lcov case with update to handle_diagnostic_positions call for discarded values

* Fix indentation of LCOV_EXCL_STOP comments

* fix amalgamation astyle issue

---------

Co-authored-by: Sush Shringarputale <sushring@linux.microsoft.com>

docs/mkdocs/docs/api/macros/json_diagnostic_positions.md

@@ -0,0 +1,61 @@
+# JSON_DIAGNOSTIC_POSITIONS
+
+```cpp
+#define JSON_DIAGNOSTIC_POSITIONS /* value */
+```
+
+This macro enables position diagnostics for generated JSON objects.
+
+When enabled, two new properties: `start_pos()` and `end_pos()` are added to `nlohmann::basic_json` objects and fields. `start_pos()` returns the start 
+position of that JSON object/field in the original string the object was parsed from. Likewise, `end_pos()` returns the end position of that JSON
+object/field in the original string the object was parsed from.
+
+`start_pos()` returns the first character of a given element in the original JSON string, while `end_pos()` returns the character following the last
+character. For objects and arrays, the first and last characters correspond to the opening or closing braces/brackets, respectively. For fields, the first
+and last character represent the opening and closing quotes or the first and last character of the field's numerical or predefined value
+(true/false/null), respectively.
+
+Given the above, `end_pos() - start_pos()` for an object or field provides the length of the string representation for that object or field, including the
+opening or closing braces, brackets, or quotes.
+
+`start_pos()` and `end_pos()` are only set if the JSON object was parsed using `parse()`. For all other cases, `std::string::npos` will be returned.
+
+Note that enabling this macro increases the size of every JSON value by two `std::size_t` fields and adds
+slight runtime overhead.
+
+## Default definition
+
+The default value is `0` (position diagnostics are switched off).
+
+```cpp
+#define JSON_DIAGNOSTIC_POSITIONS 0
+```
+
+When the macro is not defined, the library will define it to its default value.
+
+## Notes
+
+!!! hint "CMake option"
+
+    Diagnostic messages can also be controlled with the CMake option
+    [`JSON_Diagnostic_Positions`](../../integration/cmake.md#json_diagnostic_positions) (`OFF` by default)
+    which defines `JSON_DIAGNOSTIC_POSITIONS` accordingly.
+
+## Examples
+
+??? example "Example 1: retrieving positions"
+
+    ```cpp
+    --8<-- "examples/diagnostic_positions.cpp"
+    ```
+    
+    Output:
+
+    ```
+    --8<-- "examples/diagnostic_positions.output"
+    ```
+
+    The output shows the start/end positions of all the objects and fields in the JSON string.
+
+## Version history
+

docs/mkdocs/docs/integration/cmake.md

@@ -135,6 +135,9 @@ Enable CI build targets. The exact targets are used during the several CI steps
 
 Enable [extended diagnostic messages](../home/exceptions.md#extended-diagnostic-messages) by defining macro [`JSON_DIAGNOSTICS`](../api/macros/json_diagnostics.md). This option is `OFF` by default.
 
+### `JSON_Diagnostic_Positions`
+Enable position diagnostics by defining macro [`JSON_DIAGNOSTIC_POSITIONS`](../api/macros/json_diagnostic_positions.md). This option is off by default.
+
 ### `JSON_DisableEnumSerialization`
 
 Disable default `enum` serialization by defining the macro