Only define global functions qp(), htmlqp(), and html5qp() if they haven't already been defined

[jakejackson1]

Pull Request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Build-related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

If another package has already defined the global functions qp(), htmlqp(), and html5qp() a fatal PHP error will be produced.

Issue Number: #52

What is the new behavior?

The fatal error will not be produced. However, this is only a stop-gap solution for this specific circumstance, as the library isn't likely to work as expected when these global functions are called.

Long term, these functions should be deprecated and all examples updated to use the namespaced classes directly (or a new interface).

Does this introduce a breaking change?

• Yes
• No

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 4 lines in your changes missing coverage. Please review.

Project coverage is 89.28%. Comparing base (a7bebcb) to head (6f8813c).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/qp_functions.php	33.33%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main      #53      +/-   ##
============================================
- Coverage     89.45%   89.28%   -0.18%     
  Complexity     1349     1349              
============================================
  Files            26       26              
  Lines          3036     3127      +91     
============================================
+ Hits           2716     2792      +76     
- Misses          320      335      +15     

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