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/api/basic_json_document/root.md

@@ -0,0 +1,50 @@
+# <small>nlohmann::basic_json_document::</small>root
+
+```cpp
+view_type root() const noexcept;
+```
+
+Returns a view of the root value of the document.
+
+## Return value
+
+A [`view_type`](index.md#member-types) (i.e. `#!cpp basic_json_view<BasicJsonType>`) for the root value, or a
+discarded view if the document is [discarded](is_discarded.md).
+
+## Exception safety
+
+No-throw guarantee: this function never throws exceptions.
+
+## Complexity
+
+Constant.
+
+## Notes
+
+`root()` is a cheap handle into the document's index, not a copy of anything; call it as often as needed. The
+returned view is valid under the same conditions as any other view of the document -- see
+[Object inspection](../basic_json_view/index.md) -- in particular, it is invalidated by the next
+[`read()`](read.md) or [`shrink_to_fit()`](shrink_to_fit.md) on this document.
+
+## Examples
+
+??? example
+
+    ```cpp
+    --8<-- "examples/basic_json_document__root.cpp"
+    ```
+
+    Output:
+
+    ```json
+    --8<-- "examples/basic_json_document__root.output"
+    ```
+
+## See also
+
+- [is_discarded](is_discarded.md) - return whether the last parse failed
+- [materialize](../basic_json_view/materialize.md) - build the `BasicJsonType` value of a subtree
+
+## Version history
+
+- Added in version 3.13.0.