feat: simplify setGlobalPrefix method in Serinus (closes #73)

[francescovallone]

Description

The utility class will be used only for internal reference, the users can now use a simple string to manage the global prefix.

Also the method setGlobalPrefix has been changed with an actual setter called globalPrefix

Fixes #73

Type of change

Please delete options that are not relevant.

• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.26%. Comparing base (b063a69) to head (937962a).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #75      +/-   ##
===========================================
+ Coverage    89.03%   89.26%   +0.22%     
===========================================
  Files           74       74              
  Lines         2417     2449      +32     
===========================================
+ Hits          2152     2186      +34     
+ Misses         265      263       -2     

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