Store lookup name and location info in Debg table following the same structure as fontTools.feaLib. But unlike feaLib, we don’t store feature info, because:

1. it is redundant when debugging with HarfBuzz trace message as we get the script and feature info from there

2. it is misleading/incomplete as the lookup can be registered under different script/language/feature compos, but feaLib only reports one of them.

@simoncozens @justvanrossum I used the same key as feaLib, but feature info is omitted and the line numbers seem to be different (I think feaLib reports the location of the first rule in the lookup block, while fea-rs reports the location of the lookup name), would you prefer if I used a different key here? If so, I might also use a list for the location ([filename, line, column] instead of a string "filename:line:column").