Support of additional suffixes of Fortran files for preprocessing

[jvdp1]

Potential issue: I noted that the option suffixes in the section preprocess in fpm.toml is not supported by fpm. Files with suffixes other than f90 or lower(F90) are currently ignored, even if additional suffixes are provided in fpm.toml

This PR will allow a support for additional suffixes for Fortran files that should be preproccessed (e.g. with cpp).

Related with #729

[perazz]

You're right, it seems like the feature was never implemented.
I think to do so, instead of passing one more string array as a procedure argument in tens of places, it would be worth to replace the macros array in type(package_t) with complete preprocessor settings,

fpm/src/fpm_model.f90

Lines 139 to 140 in 80869ad

	!> List of macros.
	type(string_t), allocatable :: macros(:)

i.e. use the same preprocess_config_t as in the manifest

type package_t 
![...]
! type(string_t), allocatable :: macros(:) DELETE
type(preprocess_config_t) :: preprocess
![...]
end type package_t

I don't know why the original preprocess implementation had many options in the manifest not in the package, it seems like a straigthforward choice to store them all in the package settings too.

[jvdp1]

I closes this PR in favour of #982