Allow multiple extensions for assembly file types

[cosmikwolf]

🍍
Some libraries sometimes have .s and .S files that come together (like threadx for example), and it looks like the function find_build_input_file strips the extension from the file name, and then tries to re-add using the :extension: selection. It appears that there is not support for multiple extensions here.

I think it would make a lot of sense to allow someone to put a list of extensions for filenames like so:

:extension:
  :header: .h
  :source: .c
  :assembly:
    - .s
    - .S
    - .asm

[mkarlesky]

Hi again, @cosmikwolf. Yes, this is a frequently reported limitation. We are sorry about that. Everyone suggests the same solution in the project configuration. We totally agree. Supporting multiple file extensions for a single file type and doing so in the way you've suggested is in the backlog. The difficulty is simply that Ceedling began life in a much simpler form. An intentional design constraint was supporting just a single file extension per file type. I won't go into all the details as to why. That early decision along with Ceedling's early architecture made supporting multiple file extensions quite difficult to implement. With the giant 1.0.0 release almost ready to publish, we've drastically reworked big pieces of internal architecture. With those changes, adding support for multiple file extensions will be quite a bit easier in enhancements to Ceedling after 1.0.0.

[mkarlesky]

Related: #294 #765 #617