manifest: extend preprocess_config_t for external preprocessor support

[ijatinydv]

Right now fpm silently drops any [preprocess] section that isn't cpp, there's a four-line guard in add_config() at lines 320–324 that returns early with a warning if the name isn't "cpp". This PR removes that guard and adds the fields needed to describe an external preprocessor in fpm.toml.

What changed

preprocess.f90 — two things:

1. Removed the CPP-only guard in add_config(). Any preprocessor name is now accepted.
2. Added four new fields to preprocess_config_t:
   • command — the executable to invoke (e.g. "fypp"). This is the activation key: a section without command stays silently ignored, so existing [preprocess.fypp] sections in the wild don't break.
   • arguments(:) — extra CLI flags passed verbatim.
   • output-suffix — output extension, defaults to .f90. Set to .F90 for tools that emit output requiring a second CPP pass.
   • depfile — opt-in boolean for --depfile support. Defaults .false. so custom tools that don't support the flag are never passed it.

Also added is_external(), returns .true. when command is allocated. This single check will gate all external execution in the phases that follow.

Updated everything that touches fields: check() whitelist, new_preprocess_config(), dump_to_toml, load_from_toml, preprocess_is_same, destroy, add_config, and info.

test_manifest.f90 — added two tests: preprocess-external parses a full fypp config with all new fields and verifies a serialization roundtrip; preprocess-backward-compat verifies that a section without command leaves is_external() as .false. and produces no error.

Backward compatibility

Existing [preprocess.cpp] manifests work exactly as before. A [preprocess.fypp] section without command is still silently ignored same behavior as today.

This is the manifest groundwork for the external preprocessor execution engine. The execution logic, Phase A integration in build_model(), and target system changes come in follow up PRs. Raising this early so the schema can get feedback before the rest is built on top of it.

Related: #78, #308, #469