German TN fixes

[zoobereq]

What does this PR do ?

This PR implements DE TN fixes for the following issues:

• Adds support for normalizing social media tags (e.g. @zoobereq and @zoobereq.net)
• Adds support for normalizing comma-separated digit strings
• Adds support for period-separated time formats (e.g. 2.30 and 02.30)

Before your PR is "Ready for review"

Pre checks:

• [x] Have you signed your commits? Use git commit -s to sign.
• [x] Do all unittests finish successfully before sending PR?
  1. pytest or (if your machine does not have GPU) pytest --cpu from the root folder (given you marked your test cases accordingly @pytest.mark.run_only_on('CPU')).
  2. Sparrowhawk tests bash tools/text_processing_deployment/export_grammars.sh --MODE=test ...
• [x] If you are adding a new feature: Have you added test cases for both pytest and Sparrowhawk here.
• [x] Have you added __init__.py for every folder and subfolder, including data folder which has .TSV files?
• [x] Have you followed codeQL results and removed unused variables and imports (report is at the bottom of the PR in github review box) ?
• [x] Have you added the correct license header Copyright (c) 2023, NVIDIA CORPORATION & AFFILIATES. All rights reserved. to all newly added Python files?
• [x] If you copied nemo_text_processing/text_normalization/en/graph_utils.py your header's second line should be Copyright 2015 and onwards Google, Inc.. See an example here.
• [x] Remove import guards (try import: ... except: ...) if not already done.
• [x] If you added a new language or a new feature please update the NeMo documentation (lives in different repo).
• [x] Have you added your language support to tools/text_processing_deployment/pynini_export.py.

PR Type:

• [ ] New Feature
• [x] Bugfix
• [ ] Documentation
• [x] Test

If you haven't finished some of the above items you can still open "Draft" PR.