ci: auto-label PRs by changed files

[JeanExtreme002]

Summary

Adds automatic PR labeling based on which files a pull request touches.

• .github/workflows/labeler.yml — runs actions/labeler@v5 on PRs (opened, synchronize, reopened). Uses pull_request_target (consistent with lint-pr-title.yml) so it also covers fork PRs, with sync-labels: true to drop labels when matching files leave the diff.
• .github/labeler.yml — the path→label rules.

This also makes real the reference to .github/workflows/labeler.yml already present in lint-pr-title.yml.

Label rules

Label	Matches
docs	docs/**
app	PyMemoryEditor/app/**
linux	PyMemoryEditor/linux/**
win32	PyMemoryEditor/win32/**
macos	PyMemoryEditor/macos/**
lib	PyMemoryEditor/** (any change in the package)
tests	tests/**

Since lib matches all of PyMemoryEditor/**, a backend change (e.g. PyMemoryEditor/linux/) gets both its specific label and lib, as intended.

The seven labels were created in the repo with distinct colors.