[TTS] Add VietnameseCharsTokenizer
Signed-off-by: Tu [huutu12312vn@gmail.com](mailto:huutu12312vn@gmail.com)

# What does this PR do ?

Add a Vietnamese language tokenizer for TTS training

**Collection**: [TTS]

# Changelog 
- Add VietnameseCharsTokenizer
- Add unit tests for Vietnamese

# Usage
```python
from nemo.collections.common.tokenizers.text_to_speech.tts_tokenizers import VietnameseCharsTokenizer

text = "Xin chào các bạn."

tokenizer = VietnameseCharsTokenizer(
    pad_with_space=True,
)

tokens = tokenizer(text)
graphemes = tokenizer.decode(tokens)
graphemes = graphemes.replace('|', '')

print(tokens)
#  xin chào các bạn.
print(graphemes)
```

# Before your PR is "Ready for review"
**Pre checks**:
- [x] Make sure you read and followed [Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md)
- [x] Did you write any new necessary tests?
- [x] Did you add or update any necessary documentation?
- [ ] Does the PR affect components that are optional to install? (Ex: Numba, Pynini, Apex etc)
  - [ ] Reviewer: Does the PR have correct import guards for all optional libraries?
  
**PR Type**:
- [x] New Feature
- [ ] Bugfix
- [ ] Documentation
