feat: make flang-new a first-class citizen in fpm

[perazz]

Fixes #1089.

This PR makes LLVM Flang (flang-new) a first-class citizen in fpm by updating compiler identification and flag support.
Since Classic Flang is deprecated, this change makes flang resolve to the modern LLVM implementation while preserving access to the legacy compiler, now flang-classic.

Changes Made

• Both "flang" and "flang-new" now resolve to id_flang_new (LLVM Flang)
• "flang-classic" now resolves to id_flang (Classic Flang)
• id_flang compiler name changed from "flang" to "flang-classic"

Added Complete Flag Support for flang-new

Based on the official flang-new command line reference:

• Debug/optimization: -g, -O3
• Source form: -ffree-form, -ffixed-form
• Implicit typing: -fimplicit-none
• C preprocessor: -cpp
• Platform-aware -fPIC handling (disabled on Windows MSVC target)

Backward Compatibility

• Existing flang-new usage unchanged
• Legacy users can use flang-classic

cc: @rouson