Associations added using `files.assocations` lack sufficient information to distinguish between source files and header files.

VS Code has files.assocations, which allows file extensions/names to be associated with languages. We check that, to see if a file is C, C++, CUDA, or none of the above (falling back to hard-coded associations, using is_file_supported). However, VS Code doesn't provide a setting for distinguishing between source files and header files (as that's not a universal concept). Whenever we need to make that distinction, we leverage the VC common is_valid_file, with flags indicating whether we're checking for source or header - usually ftf_compiled vs ftf_header (with some additional checks for ftf_idl, which may not be needed. We associate .idl with C++, but I'm not sure if that's valid).

We've probably got some existing bugs related to is_valid_file not considering extensions/names added by files.assocation.

One thing we could do is, if is_valid_file fails but is_supported_file succeeds, we could default to considering the file a source file (not a header). If a file had previously passed is_supported_file, perhaps all checks for ftf_compiled could be changed to !ftf_header, to entire the 'neither' case is always considered a source file.

Another approach might be to add some hook in is_valid_file that falls back to is_supported_file for any unhandled (not hard-coded) extensions, and assume they're source files (not headers). That way, is_valid_file can at least return success for those instead of dropping them entirely.

Ultimately, if we want to be able to distinguish between non-standard source and header extensions, we will need a new setting for it.

Start by tracing the is_valid_file, is_supported_file, and is_file_supported entry points, including the ftf_compiled, ftf_header, and ftf_idl checks. Compare their handling of files.assocations entries with hard-coded extensions, then determine whether the chosen behavior should treat unhandled associated files as source files or require a new setting for source/header distinctions.