fix: add ExtensionDidContributes event for themes

[Aaaaash]

Types

• 🐛 Bug Fixes

Background or solution

1. 添加 ExtensionDidContributes 事件
2. 注册主题时若是已设置的自动应用
3. 主题全部注册完成后，如果已设置的主题没有安装，fallback 到默认主题

默认主题相关修改

1. 合并所有定义到 plus.json, 原因是之前定义了三层的继承关系(include), 但实际是不必要的，需要分别读取三个文件，导致加载主题时较慢

Changelog

• 修正主题 fallback 逻辑

[codecov-commenter]

Codecov Report

Merging #214 (0a7af35) into main (f3ec863) will decrease coverage by 0.00%.
The diff coverage is 70.58%.

@@            Coverage Diff             @@
##             main     #214      +/-   ##
==========================================
- Coverage   59.40%   59.39%   -0.01%     
==========================================
  Files        1180     1180              
  Lines       72358    72366       +8     
  Branches    14980    14982       +2     
==========================================
- Hits        42984    42983       -1     
- Misses      26771    26778       +7     
- Partials     2603     2605       +2     

Impacted Files	Coverage Δ
...kages/theme/src/browser/workbench.theme.service.ts	71.21% <64.28%> (-0.99%)	⬇️
packages/core-common/src/types/extension.ts	90.00% <100.00%> (+1.11%)	⬆️
...ackages/extension/src/browser/extension.service.ts	48.04% <100.00%> (+0.20%)	⬆️
...ore-browser/src/preferences/preference-provider.ts	66.89% <0.00%> (-6.90%)	⬇️
.../core-browser/src/preferences/early-preferences.ts	90.56% <0.00%> (-1.89%)	⬇️
packages/main-layout/src/browser/layout.service.ts	67.43% <0.00%> (+2.68%)	⬆️

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