Add missing function specifiers

[AndreasLrx]

Description

Huge refactoring to add missing [[nodiscard]], [[maybe_unused]], 'noexcept, [[(un)likely]]` and some documentations.

Close #171

What type of PR is this? (check all applicable)

• Refactor
• Feature
• Bug Fix
• Optimization
• Documentation Update

[optional] Minor fixes

Added/updated tests?

• New tests written
• Existing tests updated
• Tests are not required because this is a documentation update
• I need help with writing tests

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 69.76744% with 52 lines in your changes missing coverage. Please review.

Project coverage is 81.93%. Comparing base (d245348) to head (4354a3c).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/util/serialization/toml/TomlNode.hpp	33.33%	8 Missing ⚠️
src/ecstasy/integrations/event/inputs/Gamepad.hpp	50.00%	4 Missing and 2 partials ⚠️
src/ecstasy/integrations/event/events/Event.hpp	0.00%	4 Missing ⚠️
src/ecstasy/integrations/event/inputs/Keyboard.hpp	42.85%	3 Missing and 1 partial ⚠️
src/ecstasy/integrations/event/inputs/Mouse.hpp	50.00%	2 Missing and 1 partial ⚠️
src/ecstasy/storages/MapStorage.hpp	25.00%	3 Missing ⚠️
src/ecstasy/registry/Registry.hpp	60.00%	2 Missing ⚠️
src/ecstasy/storages/Instances.hpp	50.00%	0 Missing and 2 partials ⚠️
src/util/serialization/toml/TomlArrayNode.cpp	84.61%	2 Missing ⚠️
src/util/serialization/toml/TomlObjectNode.cpp	85.71%	2 Missing ⚠️
... and 16 more

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #172      +/-   ##
==========================================
- Coverage   82.59%   81.93%   -0.67%     
==========================================
  Files          64       64              
  Lines        1103     1129      +26     
  Branches      353      352       -1     
==========================================
+ Hits          911      925      +14     
- Misses        135      144       +9     
- Partials       57       60       +3     

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