Add std::format and fmt support (#5224)

* ✨ add std::format and fmt support

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

* ♻️ reorganize PR

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

* 🧛 fix build

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

* 🧛 fix build

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

* 🧛 fix build

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

---------

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

docs/mkdocs/docs/examples/format_as.cpp

@@ -0,0 +1,16 @@
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // create a JSON value
+    json j = {{"one", 1}, {"two", 2}};
+
+    // format_as() is found via argument-dependent lookup, the same way
+    // fmt::format/fmt::print would find it
+    auto j_str = format_as(j);
+
+    std::cout << j_str << std::endl;
+}

docs/mkdocs/docs/examples/format_as.output

@@ -0,0 +1 @@
+{"one":1,"two":2}

docs/mkdocs/docs/examples/std_formatter.c++20.cpp

@@ -0,0 +1,22 @@
+#include <format>
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    json j = {{"one", 1}, {"two", 2}};
+
+    // compact formatting, like dump()
+    std::cout << std::format("{}", j) << "\n\n";
+
+    // pretty-printed formatting, like dump(4)
+    std::cout << std::format("{:#}", j) << "\n\n";
+
+    // a width sets the indent, like dump(2)
+    std::cout << std::format("{:2}", j) << "\n\n";
+
+    // fill-and-align sets the indent character, like dump(4, '.')
+    std::cout << std::format("{:.>#}", j) << std::endl;
+}

docs/mkdocs/docs/examples/std_formatter.c++20.output

@@ -0,0 +1,16 @@
+{"one":1,"two":2}
+
+{
+    "one": 1,
+    "two": 2
+}
+
+{
+  "one": 1,
+  "two": 2
+}
+
+{
+...."one": 1,
+...."two": 2
+}