fix: recognize variable-width space separators

[w3lld1]

Summary

• I updated separator detection to recognize Robot Framework's two-or-more-space separators even when RIDE is configured to write four spaces.
• I kept tab separators supported and added a regression test for the reported four-space configuration with a two-space input file.

Testing

• xvfb-run -a python -m pytest -q utest/lib/robot/parsing/test_robotreader.py — 1 passed
• python -m py_compile src/robotide/lib/robot/parsing/robotreader.py utest/lib/robot/parsing/test_robotreader.py
• git diff --check origin/develop...HEAD

I also attempted the full invoke test suite locally. Its first application pass completed with 15 passed and 3 skipped, but the main run reported failures in utest/controller/test_cellinfo.py and utest/application/test_postinstaller.py before timing out while running utest/application/test_updatenotifier.py.

Fixes #3045