json_view: dump() without StringType::capacity() (#5994)

output_buffer::grow() compared the output size with m_out.capacity(),
which the StringType requirements do not include, so dump() did not
compile with the tests' alt_string. Keep what was last reserved in the
buffer itself and use it as a lower bound of the capacity instead.

Add json_view tests with alt_string (read-only and editable documents,
dump with several growth steps, save/load), and document reserve(n) and
resize(n) as the StringType requirements of basic_json_view::dump.

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

docs/mkdocs/docs/features/types/template_parameters.md

@@ -406,6 +406,7 @@ using array_t = ArrayType<basic_json, AllocatorType<basic_json>>;
 | `#!cpp operator<<(std::ostream&, const json_pointer&)`                                                                            | streamability to `#!cpp std::ostream`                                                                                                                                                        |
 | [`to_string`](../../api/basic_json/to_string.md)                                                                                  | conversion of `StringType` to `#!cpp std::string` (the function returns a `#!cpp std::string`)                                                                                               |
 | exception messages                                                                                                                | `data()` and `size()`, or `begin()` and `end()`                                                                                                                                              |
+| [`basic_json_view::dump`](../../api/basic_json_view/dump.md)                                                                      | `reserve(n)` and `resize(n)` (one argument); `capacity()` is **not** required                                                                                                                |
 
 ### Compatible types