feat(extensions)!: improve and simplify extensions system

[joshuasing]

Summary
Replaces the current extension system with an easier to use, cleaner, and simpler system.
Resolves #203

Changes

• Replace the current extension system with improved system.
• Add ChameleonLoadEvent, ChameleonEnableEvent and ChameleonDisableEvent.
• Add 100% test coverage for new extension system.
• Add 100% test coverage for Preconditions.
• Add 100% test coverage for ChameleonUtil.
• Bump version to 0.15.0-SNAPSHOT.

Checklist

• I acknowledge and agree to the terms of the Developer Certificate of Origin.
• All contributed code can be distributed under the terms of the MIT License.
• I have read the contributing guidelines and code of conduct.
• I have checked the "Allow edit from maintainers" option.
• I have added appropriate unit tests for my changes.

This pull request contains breaking changes.

[codecov]

Codecov Report

Merging #205 (c433719) into main (4960589) will increase coverage by 7.43%.
The diff coverage is 92.10%.

❗ Current head c433719 differs from pull request most recent head 39ad890. Consider uploading reports for the commit 39ad890 to get more accurate results

@@             Coverage Diff              @@
##               main     #205      +/-   ##
============================================
+ Coverage     61.86%   69.29%   +7.43%     
- Complexity      293      387      +94     
============================================
  Files            61       73      +12     
  Lines          1416     1557     +141     
  Branches         74       87      +13     
============================================
+ Hits            876     1079     +203     
+ Misses          504      443      -61     
+ Partials         36       35       -1     

Impacted Files	Coverage Δ
...ra/chameleon/adventure/mapper/AdventureMapper.java	100.00% <ø> (ø)
.../chameleon/event/common/ChameleonDisableEvent.java	0.00% <0.00%> (ø)
...a/chameleon/event/common/ChameleonEnableEvent.java	0.00% <0.00%> (ø)
...chameleon/extension/ChameleonExtensionFactory.java	0.00% <0.00%> (ø)
.../java/dev/hypera/chameleon/util/ChameleonUtil.java	100.00% <ø> (+50.00%)	⬆️
.../java/dev/hypera/chameleon/event/EventBusImpl.java	74.07% <50.00%> (-1.37%)	⬇️
.../src/main/java/dev/hypera/chameleon/Chameleon.java	66.66% <70.00%> (+42.42%)	⬆️
...era/chameleon/event/common/ChameleonLoadEvent.java	75.00% <75.00%> (ø)
...pera/chameleon/extension/ExtensionManagerImpl.java	96.42% <96.42%> (ø)
...dev/hypera/chameleon/util/graph/DirectedGraph.java	97.50% <97.50%> (ø)
... and 11 more

... and 2 files with indirect coverage changes