Allow custom file suffix

[mavam]

When generating C++ headers via flatc, all files have the suffix _generated.h. However, in many modern C++ projects we have a *.hpp extension. It's not possible to just rename the files, because the generated files include themselves and have the name hardcoded inside. It would be great to have the ability to specify the suffix explicitly.

In #5054 I suggested an option --filename-suffix that allows for customizing everything that comes after the name of the schema. For example, flatc --file-suffix .hpp would create a filename for the schema foo that has the name foo.hpp instead of foo_generated.h.

[aardappel]

Yes, an option for generated file extension would be good. I guess C++ is the only language we currently support where more than 1 extension is ever used?

[mavam]

Yeah, I think C++ is the only language with that property. For most flexibility, we could have two options:

1. --filename-suffix, which would default to _generated
2. --filename-extension, which would default to the language-specific extension

What do you think?

[aardappel]

That sounds fine to me. Though, if we haven't ever seen a reason why an other language programmer would want to change the extension, it is not worth supporting?

Either way, feel free to create a PR supporting some or all of this.

[dbaileychess]

+1 to --filename-suffix and I agree with @aardappel that the extension option is not needed, as that can be done externally via a script.

[mavam]

@dbaileychess thanks for taking over and filing a PR. Great to see this one completed so quickly!