Fix Windows compatibility issue

[yidinghao]

In biberplus.tagger.tagger_utils, the function build_variable_dictionaries does not run properly in Windows. This is because on line 21, the filenames are being parsed by splitting on '/':

# E.g. constants/suasive_verbs.txt -> suasive_verbs
file_name = constant_file.split('/')[-1].replace('.txt', '')
variables_dict[file_name] = read_in_variables(constant_file)

This code does not behave as intended on Windows, because Windows directories use '\\' instead of '/'. Thus, the variables_dict key corresponding to constants/suasive_verbs.txt is 'constants\\suasive_verbs' instead of 'suasive_verbs'. This causes several downstream errors, e.g. when running biberplus.tagger.tag_text.

To fix this issue, this pull request parses the filenames using pathlib instead of splitting on '/', thus populating variables_dict correctly on all platforms.