Add support for Timeline diagrams

[ouhammmourachid]

Fixes #45

Add support for Timeline diagrams in the codebase.

• Create a new Timeline class in mermaid/timeline.py to represent the Timeline diagram.
• Add methods to build the Mermaid diagram script for the timeline in Timeline class.
• Update mermaid/__init__.py to import the Timeline class and add it to the __all__ list.

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 38.88889% with 11 lines in your changes missing coverage. Please review.

Project coverage is 97.12%. Comparing base (974978d) to head (370c069). Report is 47 commits behind head on main.

Files with missing lines	Patch %	Lines
mermaid/timeline.py	35.29%	11 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #207 +/- ## ========================================== - Coverage 98.15% 97.12% -1.03% ========================================== Files 32 33 +1 Lines 867 871 +4 ========================================== - Hits 851 846 -5 - Misses 16 25 +9 ```

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