allow configuring index file name

[adrian-gierakowski]

index file name is hard coded for a few supported languages:

https://github.com/aziz/SublimeHyperClick/blob/7bceb7007f2dfbcd85efd367188edce1617d7ae7/hyper_click/generic_path_resolver.py#L245C9-L253

however other language might use different name for index, like nix which uses default.nix

adding index_file_name option would allow resolve_index to work for any language