refactor(core): introduce eventsubject to replace internal observable

[wzhudev]

In this PR, we (@lumixraku & me) removed the legacy reactive programming pattern Observable and Observer from the core package to use custom EventSubject instead. Not only it keeps the stop propagation functionality the old Observable provided, but also leverages RxJS.

Pull Request Checklist

• Related tickets or issues have been linked in the PR description (or missing issue).
• Naming convention is followed (do please check it especially when you created new plugins, commands and resources).
• Unit tests have been added for the changes (if applicable).
• Breaking changes have been documented (or no breaking changes introduced in this PR).

[codecov]

Codecov Report

Attention: Patch coverage is 2.57985% with 793 lines in your changes missing coverage. Please review.

Project coverage is 26.95%. Comparing base (8beb8b0) to head (548a49a).
Report is 2 commits behind head on dev.

Files	Patch %	Lines
...ender-controllers/header-move.render-controller.ts	0.00%	116 Missing ⚠️
...der-controllers/header-resize.render-controller.ts	0.00%	72 Missing ⚠️
packages/engine-render/src/scene.transformer.ts	0.00%	71 Missing ⚠️
packages/engine-render/src/base-object.ts	0.00%	55 Missing ⚠️
packages/engine-render/src/engine.ts	0.00%	36 Missing ⚠️
packages/engine-render/src/thin-scene.ts	0.00%	36 Missing ⚠️
...ender-controllers/header-menu.render-controller.ts	0.00%	36 Missing ⚠️
packages/core/src/observer/observable.ts	2.77%	35 Missing ⚠️
...ers/render-controllers/freeze.render-controller.ts	0.00%	34 Missing ⚠️
packages/engine-render/src/scene.ts	0.00%	31 Missing ⚠️
... and 53 more

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #2590      +/-   ##
==========================================
+ Coverage   26.93%   26.95%   +0.01%     
==========================================
  Files        1690     1692       +2     
  Lines       85230    85155      -75     
  Branches    17727    17736       +9     
==========================================
- Hits        22955    22951       -4     
+ Misses      62275    62204      -71     

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