Add json_document and json_view: node index, parser, and document

Add json_document and json_view, a read-only, zero-copy index of a
JSON text, as the first public slice of the zero-copy view (#5295).

A parse produces a flat array of 16-byte nodes in document order,
one per value and one per object key. Strings stay in the source
text; escaped strings are decoded into an arena. Integers are
converted while their digits are in the cache; floats keep only
their digit layout and are converted on read. Containers store the
size of their subtree, so a reader can step over one in constant
time. A document makes a handful of allocations, however many
values it has.

The parser accepts exactly what json::parse accepts, with every
combination of ignore_comments and ignore_trailing_commas, with and
without a trailing NUL, and under JSON_STRICT_NUL_HANDLING. It is
portable C++11 and does not depend on byte order.

basic_json_document adds parse, parse_copy, accept, read (reuses a
document's memory), root, is_discarded, source, owns_source,
node_count, memory_usage, and shrink_to_fit. basic_json_view adds
type, the is_* queries, operator bool, size, empty, materialize,
and source_offset. A parse error throws the same exception
basic_json::parse would throw for the same input, message and
position included.

detail::abi_config keeps JSON_STRICT_NUL_HANDLING and
JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON readable after json.hpp
undefines them, in the ABI namespace so they always match the
basic_json in use.

A NUL byte that ends a // comment is the end of the input, as in
parse() since #5696.

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

docs/mkdocs/docs/home/exceptions.md

@@ -1043,6 +1043,23 @@ MessagePack's ext type and BSON's binary subtype are each stored in a single byt
 
     This exception was added in version 3.13.0. Before that, subtypes above 255 were silently truncated modulo 256 instead of raising an error.
 
+### json.exception.out_of_range.416
+
+[`basic_json_document::parse()`](../api/basic_json_document/parse.md) and the other parsing functions of
+[`basic_json_document`](../api/basic_json_document/index.md) index a value's position in the source text in 32 bits,
+so they do not support an input of 4 GiB or more.
+
+!!! failure "Example message"
+
+    ```
+    [json.exception.out_of_range.416] input of 4 GiB or more is not supported by json_document
+    ```
+
+!!! note
+
+    This exception was added in version 3.13.0, together with [`<nlohmann/json_view.hpp>`](../features/json_view.md).
+    [`basic_json::parse()`](../api/basic_json/parse.md) has no such limit.
+
 ## Further exceptions
 
 This exception is thrown in case of errors that cannot be classified with the