local data backend: optimise for abspath lookups

[bghira]

This pull request refactors the list_files method in simpletuner/helpers/data_backend/local.py to improve file extension filtering and simplify file path handling. The main changes focus on how file extensions are matched and how file paths are constructed and returned.

File extension filtering and path handling improvements:

• Changed the file extension filtering logic to use a tuple of dot-prefixed, lowercased extensions for more robust matching with str.endswith, instead of a set of extensions without the dot.
• Simplified file path handling by removing the use of Path objects and instead using os.path.abspath and os.path.join to build absolute file paths.
• Removed the creation of Path objects for each file found, as noted in the updated docstring and code.

Code cleanup:

• Removed the unused import of Path from the file.