Add method for all files with an extension

[henriquegemignani]

In our test files, a common pattern is this:

all_dread_bsmat = [name for name in dread_data.all_name_to_asset_id().keys() if name.endswith(".bsmat")]

It'd be nice to add a method to dread_data and samus_returns_data in order to change that call into

all_dread_bsmat = dread_data.all_files_ending_with(".bsmat")