refactor(BaseCommitizen): construct Style object directly to get rid …

[bearomorphism]

…of potential type error

Description

Remove the use of merge_styles.

Additional Context

See the implementation of _MergedStyle here. They are equivalent.

https://github.com/prompt-toolkit/python-prompt-toolkit/blob/d8adbe9bfcf5f7e95b015d8ab12e2985b9a7822b/src/prompt_toolkit/styles/style.py#L365

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (v4-9-2@cc981fc). Learn more about missing BASE report.

Additional details and impacted files

@@            Coverage Diff            @@
##             v4-9-2    #1607   +/-   ##
=========================================
  Coverage          ?   98.25%           
=========================================
  Files             ?       58           
  Lines             ?     2694           
  Branches          ?        0           
=========================================
  Hits              ?     2647           
  Misses            ?       47           
  Partials          ?        0           

Flag	Coverage Δ
unittests	98.25% <100.00%> (?)

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