Support compiling pattern literal strings at compile-time

[SpriteOvO]

Implementation of #3384.

The PR is currently in a very early stage, so it is incomplete and will continue to be changed. Opening it as a draft to take notes, track progress, and early review.

API

For users, I currently expect the API to be

using namespace spdlog_literals;

// The type of the argument is `spdlog::static_pattern_formatter`.
spdlog::set_formatter("[%n] [%^%l%$] %v"_pat);

Internal

Maintainability

The code for the flag formatters remains almost unchanged. If a new flag needs to be added in the future, perhaps just add an if constexpr.

Notable Changes

• All flag formatters are moved from file pattern_formatter-inl.h to pattern_formatter.h.

  • Reason: consteval implies inline, a consteval function can't be split into a definition and a declaration, therefore all flag formatters must be visible to these consteval functions. However, they are still inside details namespace.
  • It has been done in a separate commit ea81283 to avoid diff messes.

• SPDLOG_CONSTEXPR_FUNC has been added to the explicit constructors of all flag formatters.

• For some unsure concerns, TODO comments were left.

• ... TBD

Steps

• Parse and compile flags at compile-time.
• Wrap flags tuple into static_pattern_formatter.
• Test the output. Make sure that {,static_}pattern_formatter outputs are consistent.
• Padding support.
• Name decisions. Is the current names acceptable or are there better options?
• ... TBD