Report one-character non-numeric array indices like longer ones (#5800)

* Report one-character non-numeric array indices like longer ones

A JSON pointer reference token that is not a number but has only one
character (e.g. "/a/x") was reported as out_of_range.404 ("unresolved
reference token"), because the "is not a number" check only ran for
tokens longer than one character; "/a/xy" got parse_error.109. Both now
throw parse_error.109. "-" and the empty token are still reported as
out_of_range.404. As a consequence, value(json_pointer, default) on an
array now throws for "/x" as it already did for "/xy".

Also document why ordered_map::erase's destroy/placement-new loop on
pair<const Key, T> is kept despite [basic.life]/8 before C++20.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Document the one-character array index change

Add 3.13.0 version-history entries to at, operator[], value, patch,
patch_inplace, and unflatten, and describe in exceptions.md which array
indices throw parse_error.109 and which out_of_range.404.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/home/exceptions.md

@@ -278,7 +278,9 @@ In a JSON Pointer, only `~0` and `~1` are valid escape sequences.
 
 ### json.exception.parse_error.109
 
-A JSON Pointer array index must be a number.
+A JSON Pointer array index must be a number. This exception is thrown for an array index that does not begin with a
+digit, except for `-` and the empty reference token, which throw [`out_of_range.404`](#jsonexceptionout_of_range404)
+where they cannot be resolved.
 
 !!! failure "Example messages"
 
@@ -289,6 +291,11 @@ A JSON Pointer array index must be a number.
     [json.exception.parse_error.109] parse error: array index '+1' is not a number
     ```
 
+!!! note
+
+    Before version 3.13.0, a one-character array index that is not a digit (e.g., `x`) threw
+    [`out_of_range.404`](#jsonexceptionout_of_range404) instead.
+
 ### json.exception.parse_error.110
 
 When parsing a [binary format](../features/binary_formats/index.md), the byte vector ends before the complete value has
@@ -871,7 +878,8 @@ The provided key was not found in the JSON object.
 
 ### json.exception.out_of_range.404
 
-A reference token in a JSON Pointer could not be resolved.
+A reference token in a JSON Pointer could not be resolved, for instance an array index that begins with a digit but
+contains other characters (e.g., `1a`), or `-` where it cannot be used.
 
 !!! failure "Example message"