Formatting improvements (C++26/29)

Adds compile-time checking that a format spec is valid for the argument's type.

Adds std::runtime_format for format strings only known at run time (DR).

Fixes formatting of char code units as integers ({:d}, {:x} treat them as unsigned) (DR).

Extends runtime format strings to print and println.

Adds a formatter for std::filesystem::path with quoting/escaping options.

Lets std::print write directly to the stream when arguments are locking-safe, permitting an efficient implementation (DR).

Adds std::println() with no arguments to print a blank line.

Extends the efficient std::print path to more standard types.

Fixes path presentation problems in std::format (escaping of non-representable characters).

Makes std::format usable in constant evaluation.

Renames std::runtime_format to a less misleading name.

Fixes encoding issues in std::error_code messages and adds a formatter<std::error_code>.

Fixes the default floating-point representation used by std::format.