feat: spread out blog plugin options to theme options (close #34)

[billyyyyy3320]

Summary

Relevant issue #34

Adding simple tests to protect default configuration, but I currently don't have the right to enable ci.

IMO, it makes theme config more intuitive. We probably have to deprecate modifyBlogPluginOptions. It'll be a breaking change.

What kind of change does this PR introduce? (check at least one)

• [ ] Bugfix
• [x] Feature
• [ ] Code style update
• [ ] Refactor
• [ ] Docs
• [ ] Build-related changes
• [ ] Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• [ ] Yes
• [x] No