feat: Add support for 'fortran-standard' in fpm.toml

[Adityazzzzz]

This PR implements the fortran-standard field in the [package] table (addresses Issue #1131).

Summary: Users can now specify the Fortran standard (e.g., 2018, 2008) in fpm.toml. This value is passed to the compiler using the appropriate flag (currently implemented for GFortran as -std=f).

Changes:

• src/fpm/manifest/package.f90: Added fortran_standard to package_config_t and updated parser/dump logic.
• src/fpm_model.f90: Added fortran_standard to fpm_model_t.
• src/fpm.f90:
• Updated build_model to copy the standard from manifest to model.
• Updated new_compiler_flags to append -std=f when using GFortran.

Testing:

• Created a test project with fortran-standard = "2018".
• Verified via fpm build --verbose that the flag -std=f2018 was passed to gfortran.