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.

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

Checklist:

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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