implement an utils::print() function similar to fmt::print()

this doesn't have the constexpr requirement for the format string.
also it will help porting to std::format in C++20, which doesn't
have a similar functionality either.

doc/src/Developer_code_design.rst

@@ -300,18 +300,19 @@ Formatting with the {fmt} library
 
 The LAMMPS source code includes a copy of the `{fmt} library
 <https://fmt.dev>`_, which is preferred over formatting with the
-"printf()" family of functions.  The primary reason is that it allows
-a typesafe default format for any type of supported data.  This is
+"printf()" family of functions.  The primary reason is that it allows a
+typesafe default format for any type of supported data.  This is
 particularly useful for formatting integers of a given size (32-bit or
-64-bit) which may require different format strings depending on
-compile time settings or compilers/operating systems.  Furthermore,
-{fmt} gives better performance, has more functionality, a familiar
-formatting syntax that has similarities to ``format()`` in Python, and
-provides a facility that can be used to integrate format strings and a
-variable number of arguments into custom functions in a much simpler
-way than the varargs mechanism of the C library.  Finally, {fmt} has
-been included into the C++20 language standard, so changes to adopt it
-are future-proof.
+64-bit) which may require different format strings depending on compile
+time settings or compilers/operating systems.  Furthermore, {fmt} gives
+better performance, has more functionality, a familiar formatting syntax
+that has similarities to ``format()`` in Python, and provides a facility
+that can be used to integrate format strings and a variable number of
+arguments into custom functions in a much simpler way than the varargs
+mechanism of the C library.  Finally, {fmt} has been included into the
+C++20 language standard as ``std::format()``, so changes to adopt it are
+future-proof, for as long as they are not using any extensions that are
+not (yet) included into C++.
 
 Formatted strings are frequently created by calling the
 ``fmt::format()`` function, which will return a string as a
@@ -319,11 +320,13 @@ Formatted strings are frequently created by calling the
 ``printf()``, the {fmt} library uses ``{}`` to embed format descriptors.
 In the simplest case, no additional characters are needed, as {fmt} will
 choose the default format based on the data type of the argument.
-Otherwise, the ``fmt::print()`` function may be used instead of
-``printf()`` or ``fprintf()``.  In addition, several LAMMPS output
-functions, that originally accepted a single string as argument have
-been overloaded to accept a format string with optional arguments as
-well (e.g., ``Error::all()``, ``Error::one()``, ``utils::logmesg()``).
+Otherwise, the :cpp:func:`utils::print() <LAMMPS_NS::utils::print>`
+function may be used instead of ``printf()`` or ``fprintf()``.  In
+addition, several LAMMPS output functions, that originally accepted a
+single string as argument have been overloaded to accept a format string
+with optional arguments as well (e.g., ``Error::all()``,
+``Error::one()``, :cpp:func:`utils::logmesg()
+<LAMMPS_NS::utils::logmesg>`).
 
 Summary of the {fmt} format syntax
 ==================================